// by overloading the standard java .equals method in

	// this isn't true using the default implementation of 
        // .equals.  Why?

    public static void main(String args []){

	// create an ArrayList with a capacity of 20
	ArrayList<Integer> a = new ArrayList<Integer>(20);  
	System.out.println(a);
	a.add(10);
	System.out.println(a);		
	// can we do the following? 
	a.add(0,5);
	System.out.println(a);		
	// how about:
	a.add(1,5);
	System.out.println(a);		
	// and let's try now:
	//a.add(10,5);

	// create some instances of StoresX
	StoresX x1 = new StoresX(42);
	StoresX x2 = new StoresX(10);
	StoresX x3 = new StoresX(10);
	// notice that x3 stores the same value as x2.
	// by overloading the standard java .equals method in the class
	// StoresX we get that 
	System.out.println("x2 equals x3? " + x2.equals(x3));
	// evaluates to true 
	//this isn't true using the default implementation of .equals.  Why?

	ArrayList<StoresX> xarray = new ArrayList<StoresX>();
	xarray.add(x1);
	xarray.add(x2);
	// to check if there is an object like x3 in the ArrayList: 
	System.out.println(xarray.indexOf(x3));
	// note that indexOf iteratively applies the .equals until
	// if finds an object that is identical to the given object
	// (identical under the definition provided by the .equals method).
    }

(:source lang=java:) import java.util.ArrayList;

public class ArrayListTester {

	public static void main(String args []){

		// create an ArrayList with a capacity of 20
		ArrayList<Integer> a = new ArrayList<Integer>(20);  
		System.out.println(a);
		a.add(10);
		System.out.println(a);		
		// can we do the following? 
		a.add(0,5);
		System.out.println(a);		
		// how about:
		a.add(1,5);
		System.out.println(a);		
		// and let's try now:
		//a.add(10,5);

		// create some instances of StoresX
		StoresX x1 = new StoresX(42);
		StoresX x2 = new StoresX(10);
		StoresX x3 = new StoresX(10);
		// notice that x3 stores the same value as x2.
		// by overloading the standard java .equals method in the class
		// StoresX we get that 
		System.out.println("x2 equals x3? " + x2.equals(x3));
		// evaluates to true 
		//this isn't true using the default implementation of .equals.  Why?

		ArrayList<StoresX> xarray = new ArrayList<StoresX>();
		xarray.add(x1);
		xarray.add(x2);
		// to check if there is an object like x3 in the ArrayList: 
		System.out.println(xarray.indexOf(x3));
		// note that indexOf iteratively applies the .equals until
		// if finds an object that is identical to the given object
		// (identical under the definition provided by the .equals method).
	}

}

(:sourceend:)

And here's the code for the StoresX class (look very carefully at the .equals method):

(:source lang=java:)

public class StoresX {

	private int x;

	public StoresX(int x){
		this.x = x;
	}

	public boolean equals(Object other){
		return this.x == ((StoresX)other).x;
	}

	public int getX() {
		return x;
	}

	public void setX(int x) {
		this.x = x;
	}

}

(:sourceend:)