執行緒之間可能無關，也可能互動。
而執行緒的同步機制併發執行的互動執行緒間存在與時間有關的錯誤。

public class account {

    private String name;
    private double balance;
    public account(String name) {
        this.name = name;
        this.balance=0;
    }
    public String getName() {
        return name;
    }
    public double getBalance
 
() {
        return balance;
    }
    //
    public void put(double value){
        if(value>0){
            this.balance=this.balance+value;
        }

    }
    public double  get(double value){
        if(value>0){
            if(value<=balance){
                this.balance=this.balance-value
 
;
            }else{
                value=this.balance;
                this.balance=0;
            }

        }
        return value;
    }
}

public class save extends Thread{
    private account a;
    private double value;

    public save(account a, double value) {
        this.a = a;
        this
 
.value = value;
    }

    public void run() {
        double b=a.getBalance();
        a.put(value);
        System.out.println(a.getName()+"賬戶，現有"+b+",存入"
        +value+"，餘額"+a.getBalance());
    }


}

public class fetch extends Thread{
    private account a;
    private double value;

    public fetch(account a, double value) {
        this.a = a;
        this.value = value;
    }

    public void run() {
        double b=a.getBalance();
        System.out.println(a.getName()+"賬戶，現有"+b+",取出"
        +a.get(value)+"，餘額"+a.getBalance());
    }
}

public class bank {

    public static void main(String[] args) {

//      account a1=new account("wang");
//      save s1=new save(a1,100);
//      save s1=new save(new account("wang"),100);
//      s1.start();
        account a1=new account("wang");
        new save(a1,100).start();
        new save(a1,200).start();
        new fetch(a1,300).start();

        new save(new account("xu"),100).start();
    }

}

執行結果1：
wang賬戶，現有0.0,存入100.0，餘額100.0
wang賬戶，現有100.0,取出100.0，餘額0.0
wang賬戶，現有0.0,存入200.0，餘額200.0
xu賬戶，現有0.0,存入100.0，餘額100.0
執行結果2：
wang賬戶，現有0.0,存入100.0，餘額100.0
wang賬戶，現有100.0,存入200.0，餘額300.0
xu賬戶，現有0.0,存入100.0，餘額100.0
wang賬戶，現有300.0,取出300.0，餘額0.0
由執行結果可以看出，執行緒很亂