JAVA

Question

我在使用 getter 和 setter 返回值 null 并将其替换为另一个 class 有意义的内容时遇到问题，我想知道我做错了什么。

getter 和 setter 的配置：

public class CarClass {

    private String make;
    private String model;
    private int yearOfMake;

    public void setMake(String make){
        if (make.isEmpty()){
            this.make = "";
            System.out.println("Error please input value");
        } else {
            System.out.println("Hello");
            this.make = make; 
        }
    }

    public String getMake(){
        return make;
    }

    public String getModel() {

        return model;
    }

    public void setModel(String model) {
        if (model.isEmpty()){
            this.model = "";
            System.out.println("Error, you have not entered a valid model");
        } else {
            this.model = model;
        }
    }

    public int getYearOfMake() {
        return yearOfMake;
    }

    public void setYearOfMake(int yearOfMake) {
        if (yearOfMake > 1900)
            this.yearOfMake = yearOfMake;
        else {
            this.yearOfMake = yearOfMake;
            System.out.println("Year of make is invalid");
        }
    }
}

上面的执行脚本：

public class GetterSetter {

    public static void main(String[] args) {
        CarClassTwo bmw = new CarClassTwo();
        bmw.setMake();
        System.out.println(bmw.getMake());
        CarClass benz = new CarClass();
        benz.setModel("C300");
        System.out.println(benz.getModel());
        benz.setYearOfMake(1999);
        System.out.println(benz.getYearOfMake());
        System.out.println("!!!!!!!!");
        benz.setModel("C300");
        System.out.println(benz.getModel());
        benz.setYearOfMake(1800);
        System.out.println(benz.getYearOfMake());
        System.out.println(bmw.getMake());
        System.out.println(bmw.getModel());
    }
}

如果能帮助我理解我做错了什么，将不胜感激。

Answer 1

当执行以下代码时：

CarClassTwo bmw = new CarClassTwo();
bmw.setMake();

当时

的价值观

private String make;
private String model;

默认初始化为null。

所以当bmw.setMake();执行时，执行if (make.isEmpty()){

类似于 null.isEmpty() --> 将抛出 NullPointerException。

JAVA - 使用 getter 和 setter 为空值执行条件 if 语句

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