Collections

Collections

equals() and hashcode()

• Mainly used for storing and retrieving objects from hashed collections.
• First object's hashcode is used to figure out which hash bucket the object is in
• Then the equals() method is used to compare objects in the same hash bucket.

This means that:

• equal objects MUST have the same hashcode
• unequal objects can have the same (or different) hashcode (e.g. same bucket but objects are different)
• objects having different hashcodes MUST be unequal

Collections

There are four flavours of Collections:

1. Sorted
2. Unsorted
3. Ordered -> Means that we can iterate through the collection in the same specific order every time. For e.g. all lists are ordered by index position. See example below:

package net.suhrid;

import java.util.*;

class Foo {
	private String str;
	
	public Foo(String str) {
		this.str = str;
	}
	
	public String toString() {
		return "Foo:" + str;
	}
	
	public int compareTo(Foo f) {
		return str.compareTo(f.str);
	}
}

public class HelloWorld {

	public static void main(String[] args) {
		List<String> list = new ArrayList<String>();
		list.add("D");
		list.add("A");
		list.add("C");
		list.add("B");
		
		for(Iterator<String> iter = list.iterator(); iter.hasNext();) {
			System.out.println(iter.next());
		}
		//Will print D,A,C,B each time
		
		List<Foo> fooList = new ArrayList<Foo>();
		fooList.add(new Foo("D"));
		fooList.add(new Foo("A"));
		fooList.add(new Foo("C"));
		fooList.add(new Foo("B"));
		
		for(Iterator<Foo> iter = fooList.iterator(); iter.hasNext();) {
			System.out.println(iter.next());
		}
		//Will print Foo:D,Foo:A,Foo:C,Foo:B each time
	}
}

1. Unordered ->