【笔试题】Java 易错题精选

笔试题 Java 易错题精选

1、写出下列程序的运行结果（ ）String 不变性Java 值传递

public class Test {
    public static void main(String[] args) {
        String a = "hello";
        change(a);
        System.out.println(a);
    }

    public static void change(String name) {
        name = "world";
    }
}

运行结果

```
hello
```

2、写出下列程序的运行结果（ ）String 不变性Java 值传递

public class Test {
    public void change(String str, char ch[]) {
        str = "test ok";
        ch[0] = 'g';
    }

    public static void main(String args[]) {
        String str = new String("good");
        char[] ch = {'a', 'b', 'c'};
        Test ex = new Test();
        ex.change(str, ch);
        System.out.print(str + " and ");
        System.out.print(ch);
    }
}

运行结果

```
good and gbc
```

3、写出下列程序的运行结果（ ）Integer 类缓存== 和 equals

public class Test {
    public static void main(String[] args) {
        Integer a = 1;
        Integer b = 2;
        Integer c = 3;
        Integer d = 3;
        Integer e = 321;
        Integer f = 321;
        Long g = 3L;
        Long h = 2L;
        System.out.println(c == d);
        System.out.println(e == f);
        System.out.println(c == (a + b));
        System.out.println(c.equals(a + b));
        System.out.println(g == (a + b));
        System.out.println(g.equals(a + b));
        System.out.println(g.equals(a + h));
    }
}

运行结果

```
true
false
true
true
true
false
true
```

4、写出下列程序的运行结果（ ）String 不变性Java 值传递

public class Test {
    public static void main(String[] args) {
        Test test = new Test();
        int a = 1;
        test.addInt(a);
        System.out.println(a);
        String str = "hello";
        test.addString(str);
        System.out.println(str);
        StringBuilder sb = new StringBuilder("hello");
        test.addBuilder(sb);
        System.out.println(sb.toString());
        Long m = 1L;
        Long n = 1L;
        System.out.println(m == n);
        m = 200L;
        n = 200L;
        System.out.println(m == n);
        String x = new String("hello");
        String y = "hello";
        System.out.println(x.equals(y));
        System.out.println(x == y);
    }

    public void addInt(int a) {
        a = a + 1;
    }

    public void addString(String str) {
        str = str + "world";
    }

    public void addBuilder(StringBuilder sb) {
        sb.append("world");
    }
}

运行结果

```
1
hello
helloworld
true
false
true
false
```

5、写出下列程序的运行结果（ ）intern() 方法== 和 equals

public class Test {
    public static void main(String[] args) {
        String s1 = "Monday";
        String s2 = new String("Monday");
        s2 = s2.intern();
        if (s1 == s2) {
            System.out.println("s1 == s2");
        } else {
            System.out.println("s1 != s2");
        }
        if (s1.equals(s2)) {
            System.out.println("s1 equals s2");
        } else {
            System.out.println("s1 not equals s2");
        }
    }
}

运行结果

```
s1 == s2
s1 equals s2
```

解析

讲解：java.lang.String 的 intern() 方法"abc".intern() 方法的返回值还是字符串"abc"，表面上看起来好像这个方法没什么用处。但实际上，它做了个小动作：检查字符串池里是否存在"abc"这么一个字符串，如果存在，就返回池里的字符串；如果不存在，该方法会把"abc"添加到字符串池中，然后再返回它的引用。

6、写出下列程序的运行结果（ ）多线程 run() 和 start() 方法

public class Test {
    public static void main(String[] args) {
        Thread t = new Thread() {
            public void run() {
                pong();
            }
        };
        t.run();
        System.out.print("ping");
    }

    private static void pong() {
        System.out.print("pong");
    }
}

运行结果

```
pongping
```

7、写出下列程序的运行结果（ ）switch

public class Test {
    public static void main(String[] args) {
        System.out.println(getValue(2));
    }

    public static int getValue(int i) {
        int Test = 0;
        switch (i) {
            default:
                System.out.println("default");
            case 1:
                Test = Test + i;
            case 2:
                Test = Test + i * 2;
            case 3:
                Test = Test + i * 3;
        }
        return Test;
    }
}

运行结果

```
10
```

8、写出下列程序的运行结果（ ）变量作用域

public class MeaningOfThis {
    public final int value = 4;

    public void doIt() {
        int value = 6;
        Runnable r = new Runnable() {
            public final int value = 5;

            public void run() {
                int value = 10;
                System.out.println(this.value);
            }
        };
        r.run();
    }

    public static void main(String... args) {
        MeaningOfThis m = new MeaningOfThis();
        m.doIt();
    }
}

运行结果

```
5
```

解析

结果为 5，因为 this 指的是包含它的 Runnable，而不是外面的类 MeaningOfThis。

9、对文件名为 Test.java 的 Java 代码描述正确的是（ ）String 不变性Java 值传递构造方法

class Person {
    String name = "No name";

    public Person(String nm) {
        name = nm;
    }
}

class Employee extends Person {
    String empID = "0000";

    public Employee(String id) {
        empID = id;
    }
}

public class Test {
    public static void main(String args[]) {
        Employee e = new Employee("123");
        System.out.println(e.empID);
    }
}

A. 输出: 0000

B. 输出: 123

C. 编译报错

D. 输出: No name

答案

```
C
```

解析

方案一
```java
class Person {
String name = "No name";

public Person(String nm) {
    name = nm;
}

}

class Employee extends Person {

String empID = "0000";

public Employee(String id) {
    super("Railway Employee");
    empID = id;
}

}

public class Test {

public static void main(String[] args) {

Employee employee = new Employee("123");

System.out.println(employee.empID);

System.out.println(employee.name);

}

}

**运行结果**
```java
123
Railway Employee

方案二

class Person {
    String name = "No name";

    public Person(String nm) {
        name = nm;
    }

    public Person() {
    }
}

class Employee extends Person {
    String empID = "0000";

    public Employee(String id) {
        empID = id;
    }
}

public class Test {
    public static void main(String args[]) {
        Employee e = new Employee("123");
        System.out.println(e.empID);
    }
}

运行结果

123

10、写出下列程序的运行结果（ ）空指针异常

public class NULL {
    private static void hahn() {
        System.out.println("hahn");
    }

    public static void main(String[] args) {
        NULL.hahn();
    }
}

运行结果

```
hahn
```

11、写出下列程序的运行结果（ ）溢出Java 类型转换

public class Test {
    public static void main(String[] args) {
        int a = Integer.MAX_VALUE;
        long b = a + 1;
        System.out.println(a);
        System.out.println(b);
    }
}

运行结果

```
2147483647
-2147483648
```

解决方案

修改代码
```
public class Test {
public static void main(String[] args) {
int a = Integer.MAX_VALUE;
long b = (long) a + 1;
System.out.println(a);
System.out.println(b);
}
}
```
运行结果
```
2147483647
2147483648
```

12、写出下列程序的运行结果（ ）super 关键字final 关键字

import java.util.Date;

public class Test extends Date {
    public static void main(String[] args) {
        new Test().test();
    }

    private void test() {
        System.out.println(super.getClass().getName());
        System.out.println(getClass().getName());
        System.out.println(this.getClass().getName());

        System.out.println(super.getClass().getSuperclass().getName());
        System.out.println(getClass().getSuperclass().getName());
        System.out.println(this.getClass().getSuperclass().getName());
    }
}

运行结果

```
Test
Test
Test
java.util.Date
java.util.Date
java.util.Date
```

解析

为什么 super 没有起作用呢？简单来说，super 并不能代表一个超类的引用。
因为 super 并没有代表超类的一个引用的能力，只是代表调用父类的方法而已。所以，在子类的方法中，不能这样用 System.out.println(super);也不能使用 super.super.mathod();

参考资料

• https://blog.csdn.net/f38327782/article/details/39669677