方法返回null [简单]

Question

对于入门Java，我创建了一个Door类和一个DoorTester类。基本上，我们正在试验实例变量并创建公共方法。我所做的大门类，如下所示，但我DoorTester返回 “空” 时，它看起来.getState

Door.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

您的getState()方法不返回state，它返回name：

// Create an accessor of 'state' 
public String getState() { 
    return name; // <-- Simply change this 
} 

此外，您的构造函数不设置字段。你需要做这样的事情：

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

Answer 2

你将不得不修改Door类的构造函数一样

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

其实name和state没有得到初始化。 也看到这个What is the meaning of "this" in Java?

---

修改代码段：

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; 
} 
} 

Answer 3

，我们在您Door类

重要性

public class Door { 
private String name; 
private String state; 

public void open() { 
    state = "open"; 
} 

public void close() { 
    state = "closed"; 
} 

public Door(String name, String state) { // argument passed here need to set 
// set like 
    this.name=name; 
    this.state=state; 
} 

public String getState() { 
    return name; 
} 

public void setState(String state) { 
    state = state; // you need to use this.state=state 
} 
} 

问题关键字Java