如果我们具有两个类A和B的两个对象a和b，

public class A{
    String prop1;
    String prop2;
    String prop3;
    String prop4;
    
   public A(String prop1, String prop2, String prop3, String prop4) {
        this.prop1 = prop1;
        this.prop2 = prop2;
        this.prop3 = prop3;
        this.prop4 = prop4;
    }

    public A() {
    }
   //... setters and getters
    }



    public class B{
    String prop2;
    String prop3;

public B(String prop2, String prop3) {
        this.prop1 = prop1;
        this.prop2 = prop2;
        this.prop3 = prop3;
        this.prop4 = prop4;
    }

    public B() {
    }

    }

A a = new A("prop1","prop2","prop3","prop4");
B b = new B();
b.setProp3("p");

我们如何将非空属性从B复制到a？在上面的例子中，我们如何复制不为空的prop3并忽略为空的prop2。