Quick link: Oracle Java Docs

Formatting and style


Line wrapping for if statements should generally use the 8-space rule, since conventional (4 space) indentation makes seeing the body difficult. For example:

if ((condition1 && condition2)
        || (condition3 && condition4)
        ||!(condition5 && condition6)) {

Source: Oracle


  • File: <Class>.java
  • Class: capitalised
  • Method: lowercase camelcase



for (1. initialise; 2. if condition; 4. then iterate) {
    // 3. do statement
  1. initialises variable (can be initialised elsewhere)
  2. checks condition
  3. executes statements
  4. iteration

Source: POP2 Session 1


// for-each
for (<type> <name> : arr) { 
  // make use of <name> variable;

// equivalent to normal for loop
for (int i = 0; i < arr.length; i++) { 
  <type> <name> = arr[i];
  // make use of <name> variable;;

// Example 
public static int maximum(int[] numbers){
  // defines variable
  int max = numbers[0];
  // for each iteration
  for (int num : numbers){
    if (num > max) {
      max = num;
  return max;


  • Not appropriate when you want to modify the array
for (int num : marks) {
  // only changes num, not the array element
  num = num*2; 
  • Only iterates forward over the array in single steps

Source: Geekforgeeks


  • The default keyword specifies some code to run if there is no case match
  • When Java reaches a break keyword, it breaks out of the switch block. This will stop the execution of more code and case testing inside the block.
int day = 4;

switch (day) {
  case 1:
  case 2:
  case 3:
  case 4:
  case 5:
  case 6:
  case 7:
// Outputs "Thursday" (day 4)

Source: W3School

Lambda expression

A lambda expression is a short block of code which takes in parameters and returns a value.

Lambda expressions are similar to methods, but they do not need a name and they can be implemented right in the body of a method.

Source: W3School

The simplest lambda expression contains a single parameter and an expression:

parameter -> expression

Expressions are limited. They have to immediately return a value, and they cannot contain variables, assignments or statements such as if or for.

To use more than one parameter, wrap them in parentheses:

(parameter1, parameter2) -> expression

To do more complex operations, a code block can be used with curly braces. If the lambda expression needs to return a value, then the code block should have a return statement.

(parameter1, parameter2) -> { code block }


// Some ArrayList instance: 
// e.g. ArrayList<String> lst = ArrayList<String>();
lst.removeIf( s -> lst.indexOf(s) % 2 == 0);


Subclass Constructors

Invocation of a superclass constructor must be the first line in the subclass constructor.

The syntax for calling a superclass constructor is:

  • super(); the superclass no-argument constructor is called, or
  • super(parameter list);: the superclass constructor with a matching parameter list is called.

Source: Oracles Java Tutorials

// < Child >.java
public class < Parent > {
    // attributes
    int param1;
    int param2;
    // constructor
    < Parent >( int a, int b) {
        param1 = a; 
        param2 = b;

// < Child >.java
public class < Child > extends < Parent > {
    // constructor
    < Child >( int param1, int param2) {
		super(param1, param2); // calls parent constructor

Package: java.lang

Provides classes that are fundamental to the design of the Java programming language:

  • The most important classes are Object, which is the root of the class hierarchy, and
  • Class, instances of which represent classes at run time.

The class Math provides commonly used mathematical functions such as sine, cosine, and square root. The classes String, StringBuffer, and StringBuilder similarly provide commonly used operations on character strings.

Class Throwable encompasses objects that may be thrown by the throw statement. Subclasses of Throwable represent errors and exceptions.

Class: java.lang.reflect.Array

N.B: The package java.lang.reflect provides classes and interfaces for obtaining reflective information about classes and objects.

public final class Array
extends Object

The Array class provides static methods to dynamically create and access Java arrays.

<type>[] <name> = new <type>[<size>]

In Java, all arrays are dynamically allocated.

/* In one step: 
 * <type>[] <name> = new <type>[<size>];
int[] arr = new int[20];
int[][] arr2D = new int[10][20]; //a 2D array or matrix
int[][][] arr3D = new int[10][20][10]; //a 3D array

 * In two steps: 
 * <type>[] <name>;
 * <name> = new <type>[<size>];
int[] arr;
arr = new int[20];

 * As array literal: 
 * <type>[] <name> = new <type>[]{ element, element, element, ... }; 
int[] arr = new int[]{ 1,2,3,4,5,6,7,8,9,10 }; 

Source: Geekforgeeks


Since arrays are objects in Java, we can find their length using the object property length. Instead of s.length(), which is a String method that computers the length of a String.

Source: Geekforgeeks


For detail see: obj.clone() in this doc.

Deep copy and shallow copy:

When you clone a single-dimensional array, such as Object[], a “deep copy” is performed with the new array containing copies of the original array’s elements as opposed to references.

A clone of a multi-dimensional array (like Object[][]) is a “shallow copy,” however, which is to say that it creates only a single new array with each element array a reference to an original element array, but subarrays are shared.

Source: Geekforgeeks


  • Every array type implements the interface Cloneable (Source: Geekforgeeks).
  • Since the Array class extends the Object class (see class signature below), it inherits the obj.clone() method defined in the Object class.
public final class Array
extends Object

Class: java.lang.Object

public class Object
  • Class Object is the root of the class hierarchy.
  • Every class has Object as a superclass.
  • All objects, including arrays, implement the methods of this class.


protected Object clone()
                throws CloneNotSupportedException

Creates and returns a copy of this object. The precise meaning of “copy” may depend on the class of the object.

The general intent is that, for any object x, the expression:

  • x.clone() != x will be true,
  • x.clone().getClass() == x.getClass() will be true, and
  • x.clone().equals(x) will be true.

N.B: ⚠️ But these are not absolute requirements! ⚠️

obj.equals(Object obj)

public boolean equals(Object obj)

Indicates whether some other object is “equal to” this one, true/false).

The equals method implements an equivalence relation on non-null object references:

  • It is reflexive: for any non-null reference value x, x.equals(x) should return true.
  • It is symmetric: for any non-null reference values x and y, x.equals(y) should return true if and only if y.equals(x) returns true.
  • It is transitive: for any non-null reference values x, y, and z, if x.equals(y) returns true and y.equals(z) returns true, then x.equals(z) should return true.
  • It is consistent: for any non-null reference values x and y, multiple invocations of x.equals(y) consistently return true or consistently return false, provided no information used in equals comparisons on the objects is modified.
  • For any non-null reference value x, x.equals(null) should return false.

@Override obj.equals(Object obj)

@Override // overrides Object equals() method (for my own objects definition)
public boolean equals(Object obj) {
        // reflexivity
        if(this == obj) { return true; } 
        // comparing apples and pears
        if((obj == null) || (obj.getClass() != this.getClass())) { return false; }
        // Can now safely cast Pair type onto obj
        Pair otherPair = (Pair) obj;
        // equal is x and y are equal
        return x == otherPair.get_x() && y == otherPair.get_y();

public int hashCode() { 
    int hash = 7;
    return x * hash - y * hash;

@Override obj.hashCode(Object obj)

When we override equals(), it is recommended to also override the hashCode() method. If we don’t do so, equal objects may get different hash-values; and hash based collections, including HashMap, HashSet, and Hashtable do not work properly.

Source: Geekforgeeks

public int hashCode() { 
    int hash = 7;
    return x * hash - y * hash;


public final Class<?> getClass()

Returns the runtime class of this Object.


public String toString()

Returns a string representation of the object.

In general, the toString method returns a string that “textually represents” this object:

  • The result should be a concise but informative representation that is easy for a person to read.
  • It is recommended that all subclasses override this method.

Class: java.lang.Character

The Character class wraps a value of the primitive type char in an object. An object of class Character contains a single field whose type is char.

Two parameter types:

  • int codepoint (unicode)
  • char ch (character)

getType(char ch)


public static boolean isAlphabetic(int codePoint)

Mostly used with s.codePointAt(i) e.g.


Character.isLetter(char ch)

public static boolean isLetter(char ch)

A character is considered to be a letter if its general category type, provided by Character.getType(ch), is any of the following:


Character.isDigit(char ch)

public static boolean isDigit(char ch)

A character is a digit if its general category type, provided by Character.getType(ch), is DECIMAL_DIGIT_NUMBER.

Character.isLowerCase(char ch)

public static boolean isLowerCase(char ch)

A character is lowercase if its general category type, provided by Character.getType(ch), is LOWERCASE_LETTER

Character.isUpperCase(char ch)

public static boolean isUpperCase(char ch)

A character is uppercase if its general category type, provided by Character.getType(ch), is UPPERCASE_LETTER.

Character.isWhitespace(char ch)

public static boolean isWhitespace(char ch)

A character is a Java whitespace character if and only if it satisfies one of the following criteria: It is a Unicode space character (SPACE_SEPARATOR, LINE_SEPARATOR, or PARAGRAPH_SEPARATOR) but is not also a non-breaking space (’\u00A0’, ‘\u2007’, ‘\u202F’).

  • It is ‘\t’, U+0009 HORIZONTAL TABULATION.
  • It is ‘\n’, U+000A LINE FEED.
  • It is ‘\u000B’, U+000B VERTICAL TABULATION.
  • It is ‘\f’, U+000C FORM FEED.
  • It is ‘\r’, U+000D CARRIAGE RETURN.
  • It is ‘\u001C’, U+001C FILE SEPARATOR.
  • It is ‘\u001D’, U+001D GROUP SEPARATOR.
  • It is ‘\u001E’, U+001E RECORD SEPARATOR.
  • It is ‘\u001F’, U+001F UNIT SEPARATOR.

Character.toLowerCase(char ch)

public static char toLowerCase(char ch)

Converts the character argument to lowercase using case mapping information from the UnicodeData file.

Character.toUpperCase(char ch)

public static char toUpperCase(char ch)

Converts the character argument to uppercase using case mapping information from the UnicodeData file.

Class: java.lang.String

The String class represents character strings. All string literals in Java programs, such as “abc”, are implemented as instances of this class.

Strings are constant (immutable); their values cannot be changed after they are created.

s.length(int index)

public char charAt(int index)

Returns the length of this string. The length is equal to the number of Unicode code units in the string.

s.charAt(int index)

public char charAt(int index)

Returns the char value at the specified index. An index ranges from 0 to length() - 1.


public int indexOf(int ch)

Returns the index within this string of the first occurrence of the specified character ch.

public int indexOf(int ch,
                   int fromIndex)

Returns the index within this string of the first occurrence of the specified character, starting the search at the specified index.

s.codePointAt(int index)

public int codePointAt(int index)       

Returns the character (Unicode code point) at the specified index. The index refers to char values (Unicode code units) and ranges from 0 to length() - 1.


public String toLowerCase()

Converts all of the characters in this String to lower case using the rules of the default locale.


public String toUpperCase()

Converts all of the characters in this String to upper case using the rules of the default locale.


public int compareTo(String anotherString)

Compares two strings lexicographically. The comparison is based on the Unicode value of each character in the strings.

  • negative integer: if s lexicographically precedes the argument string s2
  • positive integer: if s lexicographically follows the argument string s2
  • 0: when the s.equals(s2) returns true.

Class: java.lang.StringBuilder

public final class StringBuilder
extends Object
implements Serializable, Comparable<StringBuilder>, CharSequence

A mutable sequence of characters. The principal operations on a StringBuilder are the append (at end) and insert (at index) methods, which are overloaded so as to accept data of any type.

If sb refers to an instance of a StringBuilder, then sb.append(x) has the same effect as sb.insert(sb.length(), x).

Use sb.toString() to convert the object into an immutable String.

Constructor: sb.StringBuilder() sb.StringBuilder(String str)

Constructs a string builder initialized to the contents of the specified string.

StringBuilder sbEmpty = new StringBuilder();
StringBuilder sbExisting = new StringBuilder("Hello");


Returns a string representing the data in this sequence.

sb.append(Object obj)

Appends the string representation of the Object argument.

Exists with various methods signatures:

  • append(boolean b): Appends the string representation of the boolean argument to the sequence.
  • append(char c): Appends the string representation of the char argument to this sequence.
  • append(double d): Appends the string representation of the double argument to this sequence.
  • append(float f): Appends the string representation of the float argument to this sequence.
  • append(int i): Appends the string representation of the int argument to this sequence.
  • append(long lng): Appends the string representation of the long argument to this sequence.
  • append(Object obj): Appends the string representation of the Object argument.
  • append(String str): Appends the specified string to this character sequence.

sb.delete(int start, int end)

Removes the characters in a substring of this sequence. The substring begins at the specified start and extends to the character at index end - 1 or to the end of the sequence if no such character exists. If start is equal to end, no changes are made.

sb.deleteCharAt(int index)

public StringBuilder deleteCharAt(int index)

Removes the char at the specified position in this sequence. This sequence is shortened by one char.

sb.replace(int start, int end, String str)

public StringBuilder replace​(int start, int end, String str)


  • start - The beginning index, inclusive.
  • end - The ending index, exclusive.
  • str - String that will replace previous contents.

Package: java.util

Contains the collections framework, some internationalization support classes, a service loader, properties, random number generation, string parsing and scanning classes, base64 encoding and decoding, a bit array, and several miscellaneous utility classes.

Class: ArrayList<E>

public class ArrayList<E>
extends AbstractList<E>
implements List<E>, RandomAccess, Cloneable, Serializable
  • <E> = the type of elements in this list

Resizable-array implementation of the List interface. Implements all optional list operations, and permits all elements, including null. (This class is roughly equivalent to Vector, except that it is unsynchronized.)

Constructor: ArrayList()

ArrayList<String> lString = new ArrayList<String>()
ArrayList<Integer> lInteger = new ArrayList<Integer>()

Constructs an empty list with an initial capacity of ten.

Elements in an ArrayList are actually objects.

To use other types, such as int, you must specify an equivalent wrapper class: Integer. For other primitive types, use: Boolean for boolean, Character for char, Double for double, etc:

Source: W3School


public int size()

Returns the number of elements in this list.

lst.remove(int index)

Removes the element at the specified position in this list. Shifts any subsequent elements to the left (subtracts one from their indices).

public E remove(int index)


See in Interface Colletion<E> method c.removeIf(Predicate<?superE>filter) in this doc.

lst.indexOf(Object o)

public int indexOf(Object o)

Returns the index of the first occurrence of the specified element in this list, or -1 if this list does not contain the element.

Class: Scanner

A simple text scanner which can parse primitive types and strings using regular expressions.

A Scanner breaks its input into tokens using a delimiter pattern, which by default matches whitespace. The resulting tokens may then be converted into values of different types using the various next methods.

The default whitespace delimiter used by a scanner is as recognized by Character.isWhitespace().


  • Scanner(File source): Constructs a new Scanner that produces values scanned from the specified file.
  • Scanner(String source): Constructs a new Scanner that produces values scanned from the specified input stream.
  • Scanner(InputStream source): Constructs a new Scanner that produces values scanned from the specified string.
  • (more available)

sc.useDelimiter(String pattern)

Sets this scanner’s delimiting pattern to a pattern constructed from the specified String.

String input = "1 fish 2 fish red fish blue fish";
Scanner sc = new Scanner(input).useDelimiter("\\s*fish\\s*");


Closes this scanner.

sc.hasNext() variations

  • sc.hasNextInt()
  • sc.hasNextLine()
  • sc.hasNext(String pattern): Returns true if the next token matches the pattern constructed from the specified string.


Finds and returns the next complete token from this scanner.

  • sc.next(String pattern): Returns the next token if it matches the pattern constructed from the specified string.
  • [sc.nextInt()]: Scans the next token of the input as an int.
  • sc.nextLine(): Advances this scanner past the current line and returns the input that was skipped.

Interface: Collection<E>

public interface Collection<E>
extends Iterable<E>

The root interface in the collection hierarchy.

c.removeIf(Predicate<? super E> filter)

Removes all of the elements of this collection that satisfy the given predicate.

For context, Interface Predicate<T> is a functional interface and can therefore be used as the assignment target for a lambda expression or method reference.

Interface: Map<K,V>

  • An object that maps keys to values.
  • A map cannot contain duplicate keys;
  • each key can map to at most one value.

Type Parameters:

  • K: the type of keys maintained by this map
  • V: the type of mapped values

This interface takes the place of the Dictionary class, which was a totally abstract class rather than an interface.

Class: HashMap<K,V>

public class HashMap<K,V>
extends AbstractMap<K,V>
implements Map<K,V>, Cloneable, Serializable

This implementation provides constant-time performance for the basic operations (get and put), assuming the hash function disperses the elements properly among the buckets.

Useful methods:

  • m.get(Object key)
  • m.put(K key, V value)
  • m.size()
  • m.remove(Object key)
  • m.clear()
  • m.clone()
  • m.containsKey(Object key)
  • m.containsValue(Object value)
  • m.entrySet() returns Set<Map.Entry<K,V>>
  • m.keySet() returns Set<K>

For each Map.Entry pair in entrySet()

// HashMap<K, V> m = new HashMap();

for (Map.Entry<K, V> pair: m.entrySet()) {
    // pair.getKey()
    // pair.getValue();

Source: zetcode.com

Iterator over entrySet()

// HashMap<K, V> m = new HashMap();

Iterator< Map.Entry<K, V> > it = items.entrySet().iterator();

while (it.hasNext()) {
    Map.Entry<K, V> pair = it.next();
    // pair.getKey()
    // pair.getValue();

Source: zetcode.com

Interface: Set<E>

public interface Set<E>
extends Collection<E>

A collection that contains no duplicate elements. More formally, sets contain no pair of elements e1 and e2 such that e1.equals(e2), and at most one null element.

All known implementing classes:

  • AbstractSet
  • HashSet
  • LinkedHashSet
  • TreeSet
  • more excl. for simplicity

Class: HashSet<E>

public class HashSet<E>
extends AbstractSet<E>
implements Set<E>, Cloneable, Serializable

This class implements the Set interface, backed by a hash table (actually a HashMap instance). It makes no guarantees as to the iteration order of the set; in particular, it does not guarantee that the order will remain constant over time. This class permits the null element.

Set s = new Hashset()

Constructs a new, empty set.

Set s = new HashSet(Collection<? extends E> c)

Constructs a new set containing the elements in the specified collection c.

s.add(E e)

public boolean add(E e)

Adds the specified element to this set if it is not already present.

If this set already contains the element, the call leaves the set unchanged and returns false.


public void clear()

Removes all of the elements from this set. The set will be empty after this call returns.


public Object clone()

Returns a shallow copy of this HashSet instance: the elements themselves are not cloned.

contains(Object o)

public boolean contains(Object o)

Returns true if this set contains the specified element.


public boolean isEmpty()

Returns true if this set contains no elements.

s.remove(Object o)

public boolean remove(Object o)

Removes the specified element from this set if it is present.

Returns true if this set contained the element (or equivalently, if this set changed as a result of the call).


public int size()

Returns the number of elements in this set (its cardinality).


public Object[] toArray()

Returns an array containing all of the elements in this collection.