java - 方法返回 null [简单]

Question

对于 Java 入门，我创建了一个 Door 类和一个 DoorTester 类。本质上，我们正在尝试实例变量并创建公共(public)方法。我制作了门类，如下所示，但我的 DoorTester 在查找 .getState 时返回“null”

门.java

public class Door {
// Create instance variables of type String
private String name;
private String state;

// Declare method 'open' and 'close'
public void open() {
    state = "open";
}
public void close() {
    state = "closed";
}

// Add a constructor for the Door class
public Door(String name, String state) {
}

// Create an accessor of 'state'
public String getState() {
    return name;
}

// Set the state
public void setState(String newState) {
    state = newState;
}

}

DoorTester.java

public class DoorTester {
public static void main(String[] args) {
   Door frontDoor = new Door("Front", "open");
   System.out.println("The front door is " + frontDoor.getState());
   System.out.println("Expected:  open");
   Door backDoor = new Door("Back", "closed");
   System.out.println("Expected:  closed");        
   // Use the mutator to change the state variable
   backDoor.setState("open");
   System.out.println("The back door is " + backDoor.getState());
   System.out.println("Expected:  open");
   // Add code to test the setName mutator here 
  }

}

Answer 1

您必须修改 Door 类的构造函数，例如

public Door(String name, String state) {
this.name=name;
this.state=state;
}

实际上name和state没有被初始化。另请参阅此 What is the meaning of "this" in Java?

<小时/>

修改后的代码片段:

public class Door {
// Create instance variables of type String
private String name;
private String state;

// Declare method 'open' and 'close'
public void open() {
    state = "open";
}
public void close() {
    state = "closed";
}

// Add a constructor for the Door class
public Door(String name, String state) {
 this.name=name;
 this.state=state;
}

// Create an accessor of 'state'
public String getState() {
    return state;            //<<<<<<<----------also make an Edit here 
}

// Set the state
public void setState(String newState) {
    state = newState;
}
}