Question

我有两个班级：

public class ClassA {

String NewName;

public ClassA() {
    System.out.println("new name in A constructor: " + NewName);
}

public static void main(String[] args) {
    ClassA aM = new ClassA();
    aM.action();
    aM.displayNewName();
}

public void displayNewName() {
    System.out.println("new name in display method:" + getNewName());
}

public void action() {
    ClassB bA = new ClassB();
    System.out.println("new name in Action:" + getNewName());
}

public String getNewName() {
    return NewName;
}

public void setNewName(String newN) {
    NewName = newN;
    System.out.println("new name in A Set Method: " + getNewName());
}
}

和

public class ClassB {

String newName = "jack";
ClassA a;

public ClassB(){
    a.setNewName(newName);
}
}

我无法在jack方法上获得ClassA值（例如displayNewName()和action方法。

更新

    public class ClassB {

String newName = "jack";
ClassA a = new ClassA();

public ClassB(){
    a.setNewName(newName);
}
}

输出：

new name in A constructor: null
new name in A constructor: null
new name in A Set Method: jack
new name in Action:null
new name in display method:null

Answer 1

变量a未初始化。它将抛出nullpointerexception。你需要在构造函数中添加以下行来初始化它：a = new ClassA();

<强> EDIT1：

根据我的理解，你需要这样做：

public class ClassB {

String newName = "jack";

public ClassB(ClassA a){
    a.setNewName(newName);
}
}

然后在ClassB中修改action（）

public void action() {
    ClassB bA = new ClassB(this);
    System.out.println("new name in Action:" + getNewName());
}

Answer 2

更改ClassB

public class ClassB {

String newName = "jack";
ClassA a=new ClassA();  // Initialize a

public ClassB(){
    a.setNewName(newName);
}
}

编辑：

public void action() {
        ClassB bA = new ClassB();
        System.out.println("new name in Action:" + bA.newName);
    }

Answer 3

a未初始化。尝试a = new ClassA（）;在构造函数

中

由两个类构造函数接收新值

3 个答案: