public class MyThread extends Thread{
    public static int num = 0;
    public void run(){
        add();  //根据题目意思不同线程 调用 add 或 del
    }
    public synchronized void math(int type){
        switch(type){
            case 1:
                ++num;
                break;
            case 2:
                --num;
                break;
        }
    }
    public void add(){
        math(1);
    }
    public void del(){
        math(2);
    }
}

------解决方案--------------------
public class Test {
  static int j = 0;
  public static void main(String[] args){
    
 MyThread1 mt1 = new MyThread1();
 mt1.start();
 MyThread1 mt2 = new MyThread1();
 mt2.start();
 MyThread2 mt3 = new MyThread2();
 mt3.start();
 MyThread2 mt4 = new MyThread2();
 mt4.start();
  }
  static class MyThread1 extends Thread{
  public void run(){
   while(true){
   try{
   j = ++j;
   this.sleep(1000);
   System.out.println("此时j的值是"+j);
   }catch(InterruptedException e){
    e.printStackTrace();
    }
   }
  }
 }
  static class MyThread2 extends Thread{
  public void run(){
   while(true){
   try{
   j = --j;
   this.sleep(1000);
   System.out.println("此时j的值是"+j);
   }catch(InterruptedException e){
    e.printStackTrace();
   }
  }
 }
  }


------解决方案--------------------
第二题  第三问不明白...
Java code

import java.util.HashMap;
public class H extends HashMap {
    private final int size = 2;
    public Object put(Object key, Object value, long nas) {
        if (this.size() > size) {//设置最大长度
            throw new IllegalStateException();
        }
        synchronized (this) {
            try {
                this.wait(nas);//设置延迟插入
            } catch (Exception e) {
                e.printStackTrace();
            }
            return super.put(key, value);
        }
    }
}
class Test {
    public static void main(String[] args) {
        H m = new H();
        m.put("s", "ss", 1000L);
        m.put("d", "ss", 1000L);
        m.put("F", "ss", 1000L);
        m.put("f", "ss", 1000L);
        System.out.println(m.get("s"));
    }
}

------解决方案--------------------
Java code

public class TestThread {
        private int j; 
        public TestThread(int j) {this.j = j;} 
        private synchronized void inc(){
            j++;
            System.out.println(j + "--Inc--" + 
                               Thread.currentThread().getName());
        }
        private synchronized void dec(){
            j--;
            System.out.println(j + "--Dec--" + 
                               Thread.currentThread().getName());
        }
        public void run() {
            (new Dec()).start();
            new Thread(new Inc()).start();
            (new Dec()).start();
            new Thread(new Inc()).start();
        }
        class Dec extends Thread {
            public void run() {
                for(int i=0; i<100; i++){
                    dec();    
                }
            }
        }
        class Inc implements Runnable {
            public void run() {
                for(int i=0; i<100; i++){
                    inc();