Almost all the types in Java

Java Data Types - group of objects

Basic Examples

// example.java

import java.lang.Boolean;
import java.lang.Double;
import java.lang.Float;
import java.lang.Integer;
import java.lang.Long;
import java.lang.String;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.HashSet;
import java.util.LinkedList;
import java.util.Map;
import java.util.Set;

public class Example {
    public static void main(String[] args) {
        // Primitive types
        byte myByte = 127; // 8-bit, 2^8, -128 to 127
        short myShort = 32767; // 16-bit, 2^16. -32768 to 32767
        int myInt = 2147483647; // 32-bit, 2^32, -2147483648 to -2147483647
        long myLong = 9223372036854775807L; // 64-bit, 2^64, -9223372036854775808 to 9223372036854775807L, L otherwise
        float myFloat = 3.14f; // 32-bit, 2^32, 
        double myDouble = 3.141592653589793; // 64-bit, 2^64
        char myChar = 'A'; // 16-bit, 2^16, 
        boolean myBoolean = true; // 1-bit, 2^1

        // Reference types
        String myString = "Hello, world!";
        Class myClass = String.class;
        Object myObject = new Object();

        // Collection data types
        ArrayList<String> myArrayList = new ArrayList<>();
        myArrayList.add("Apple");
        myArrayList.add("Orange");
        myArrayList.add("Banana");

        LinkedList<String> myLinkedList = new LinkedList<>();
        myLinkedList.add("Apple");
        myLinkedList.add("Orange");
        myLinkedList.add("Banana");

        Set<String> mySet = new HashSet<>();
        mySet.add("Apple");
        mySet.add("Orange");
        mySet.add("Banana");

        Map<String, Integer> myMap = new HashMap<>();
        myMap.put("Apple", 1);
        myMap.put("Orange", 2);
        myMap.put("Banana", 3);

        // Print out the result
        System.out.println("myByte: " + myByte);
        System.out.println("myShort: " + myShort);
        System.out.println("myInt: " + myInt);
        System.out.println("myLong: " + myLong);
        System.out.println("myFloat: " + myFloat);
        System.out.println("myDouble: " + myDouble);
        System.out.println("myChar: " + myChar);
        System.out.println("myBoolean: " + myBoolean);

        System.out.println("myString: " + myString);
        System.out.println("myClass: " + myClass);
        System.out.println("myObject: " + myObject);

        System.out.println("myArrayList: " + myArrayList);
        System.out.println("myLinkedList: " + myLinkedList);
        System.out.println("mySet: " + mySet);
        System.out.println("myMap: " + myMap);
    }
}

Results

$ java ./example.java
myByte: 127
myShort: 32767
myInt: 2147483647
myLong: 9223372036854775807
myFloat: 3.14
myDouble: 3.141592653589793
myChar: A
myBoolean: true
myString: Hello, world!
myClass: class java.lang.String
myObject: java.lang.Object@3745e5c6
myArrayList: [Apple, Orange, Banana]
myLinkedList: [Apple, Orange, Banana]
mySet: [Apple, Orange, Banana]
myMap: {Apple=1, Orange=2, Banana=3}

1. Set <-- HashSet

In 3Sum, we use hashset to improve lookup time for duplicate values.

...
Set<List<Integer>> set = new HashSet<>();
set.add(Arrays.asList(1, 2, 3)); 
return new ArrayList<>(set);
...

HashSet is a class that implements the Set interface in Java

Methods

Set<List<Integer>> set = new HashSet<>(); // slow, less memory used
HashSet<List<Integer>> set = new HashSet<>(); // fast, more memory

Set = abstract class with basic functionality
HashSet = concrete class that implements Set interface

set.iteractor()/toArray()/add()/remove()/contains()/size()/isEmpty()
hashset.add()/remove()/contains()/size()/isEmpty()

Convert ArrayList to/from HashSet

HashSet<String> hashSet = new HashSet<>();
hashSet.add("why so many types");
ArrayList<String> arrayList = new ArrayList<>(hashSet);

HashSet<String> hashSet = new HashSet<>();
hashSet.add("help my brain");
ArrayList<String> arrayList = (ArrayList<String>) hashSet.toArray();

Others

I will update them in the future. My brain is on max CPU at the moment.