You write the following class:

public class ChangeableInt {
	private static int num;
	
	public ChangeableInt(int tempNum){
		num = tempNum;
	}
	
	public int getNum(){
		return num;
	}
	
	public void setNum(int newNum){
		num = newNum;
	}
	
	public void increment(){
		num++;
	}
}

which implements a mutable integer.

Like a good programmer, you write some tests to make sure your class is working as expected:

public static void test1(){
	ChangeableInt num = new ChangeableInt(10);
	System.out.println(num.getNum());
	
	num.increment();
	System.out.println(num.getNum());
	
	num.setNum(20);
	System.out.println(num.getNum());
}
	
public static void test2(){
	ChangeableInt num1 = new ChangeableInt(10);
	ChangeableInt num2 = new ChangeableInt(20);
		
	System.out.println(num1.getNum());
	System.out.println(num2.getNum());
}

Everything looks good when you run the first test and you get the expected output of:
10
11
20

However, when you run the second test, you get a surprising result:
20
20

What is the problem?