【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

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.****.net/f38327782/article/details/39669677

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【笔试题】Java 易错题精选

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