Java 经典笔试题目整理(一)

文章目录

• ​​Question 1 - ++i & finally​​
• ​​Question 2 - 值传递和引用传递​​
• ​​Question 3 - 类的加载顺序​​
• ​​Question 4 - SpringMvc中获取Request​​

Question 1 - ++i & finally

public static void main(String[] args) {

  System.out.println("result: " + calculate());

}

public static int calculate(){
  int i = 0;
  try {
    ++i;
  } finally {
    ++i;
    System.out.println("i: " + i);
  }
  return ++i;
}

i: 2
result: 3

public static void main(String[] args) {

  System.out.println("result: " + calculate());

}

public static int calculate(){
  int i = 0;
  try {
    ++i;
    return ++i;
  } finally {
    ++i;
    System.out.println("i: " + i);
  }
}

i: 3
result: 2

Question 2 - 值传递和引用传递

public static void main(String[] args) {
    StringBuffer x = new StringBuffer("a");
    StringBuffer y = new StringBuffer("b");
    change(x, y);
    System.out.println(x);
    System.out.println(y);


    String m = "m";
    String n = "n";
    change(m, n);
    System.out.println(m);
    System.out.println(n);
}

public static void change(StringBuffer x, StringBuffer y) {
    x.append(y);
    y = x;
    System.out.println("StringBuffer: " + y);
}

public static void change(String x, String y) {
    x = x + y;
    y = x;
    System.out.println("String: " + y);
}

StringBuffer: ab
ab
b
String: mn
m
n

Question 3 - 类的加载顺序

public class B extends A {
    private String address = address();

    {
        System.out.println("B none static code block.");
    }

    private static int sex = sex();

    static {
        System.out.println("B static code block.");
    }

    public B() {
        System.out.println("B Constructor init.");
    }
    
    public B(String address) {
        this.address = address;
        System.out.println("B Constructor(address) init.");
    }

    private String address() {
        System.out.println("B property address");
        return "address";
    }

    private static int sex() {
        System.out.println("B static property sex");
        return 1;
    }

    public static void main(String[] args) {

        System.out.println("main start...");

        new B();

    }

}


class A {
    private String name = name();

    private static int count = count();

    {
        System.out.println("A none static code block.");
    }

    static {
        System.out.println("A static code block.");
    }

    public A() {
        System.out.println("A Constructor init.");
    }

    private String name() {
        System.out.println("A property name");
        return "name";
    }

    private static int count() {
        System.out.println("A static property count");
        return 1;
    }

}

A static property count
A static code block.
B static property sex
B static code block.
main start…
A property name
A none static code block.
A Constructor init.
B property address
B none static code block.
B Constructor init.

​​类的加载顺序​​

1. 静态： 父类优先， 静态变量和代码块按照顺序加载
2. 非静态： 父类优先， 成员变量和代码块按照顺序加载
3. 构造函数： 父类优先

Question 4 - SpringMvc中获取Request

• Controller 中加参数
• 自动注入
• 手动调用
• @ModelAttribute 方法

// Controller 中加参数
@Controller
public class AALController {
    @RequestMapping("/answer")
    public void answer(HttpServletRequest request) {
        ......
    }
}

// 自动注入
@Controller
public class AALController {
     // 自动注入 request
    @Autowired
    private HttpServletRequest request; 
     
    @RequestMapping("/answer")
    public void answer() throws InterruptedException{
        ......
    }
}

// 手动调用
@Controller
public class AALController {
    @RequestMapping("/answer")
    public void answer() throws InterruptedException {
        HttpServletRequest request = ((ServletRequestAttributes) (RequestContextHolder.currentRequestAttributes())).getRequest();
            .......
    }
}

// @ModelAttribute 方法
@Controller
public class AALController {
    // 此处 request  非线程安全
    private HttpServletRequest request; 
    @ModelAttribute
    public void bindRequest(HttpServletRequest request) {
        // 此处 request 线程安全
        this.request = request;  
    }
    @RequestMapping("/answer")
    public void answer() throws InterruptedException {
        ......
    }
}