为什么参考变量在本例中引用不同的值？

public class AAA {
    public static void main(String[] args) {
        Integer bbb, aaa = new Integer(5);
        System.out.print(aaa);
        bbb = aaa;
        System.out.print(bbb);
        aaa = 4;  
        System.out.print(aaa);
        System.out.print(bbb);
    }
}
public class AAA {
    public static void main(