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class Test {
	String x= "hello";
}

// make an object
Test myTest = new Test();

Test = name of the class
myTest = object reference variable
Test() = contractor of test

Credit: screen shot from SlideNerd videos on youtube

myTest does not actually hold an object, it holds a REFERENCE to an abject, which is a memory location. The object itself is stored somewhere else – this is for efficiency of memory usage.

System.out.println(myTest);

will produce something like

Test@4u254por

4u254por is the memory location of the actual Test object.

If we say:

Test myTest = new Test();
Test yourTest = myTest;

Credit: screen shot from SlideNerd videos on youtube

we are saying yourTest points to the same memory address as myTest.

PRIMITIVE TYPES

int y=15;

y and 15 are in the same place in memory. This is because primitive types require little storage to the reference and the value are stored in the same place.

Credit: screen shot from SlideNerd videos on youtube

When you copy primitive types, the original and copy are independent values.

Take a look at the following:

Credit: screen shot from SlideNerd videos on youtube

Here Vivs gives a good example using objects. Unlike primitive types, with objects, if we use =, and then use a method to change the variable in one object, change(), both object yourTest and myTest contain the same value of x.

Here is a code example of this:

public class TestMain {

	public static void main(String[] args) {
	
		
		// primitive data types
		int x=20;
		int y=10;
		System.out.println("x= "+x +" y= "+y);
		x=y;
		System.out.println("x= "+x +" y= "+y);
		y=5;
		System.out.println("x= "+x +" y= "+y);
		
		
		// object references
		Test myTest = new Test();
		System.out.println("myTest= "+myTest.message);
		Test anotherTest = new Test();
		System.out.println("anotherTest= "+anotherTest.message);
		
		Test copiedTest = myTest;
		System.out.println("copiedTest= "+copiedTest.message);
		
		copiedTest.change();
		System.out.println("myTest= "+myTest.message);
		System.out.println("anotherTest= "+anotherTest.message);
		System.out.println("copiedTest= "+copiedTest.message);
	}
}


class Test
{
	String message="hello!";
	
	public void change()
	{
		message="good bye";
	}

}

Outputs:

x= 20 y= 10
x= 10 y= 10
x= 10 y= 5
myTest= hello!
anotherTest= hello!
copiedTest= hello!
myTest= good bye
anotherTest= hello!
copiedTest= good bye