Java multithread error `java.lang.IllegalMonitorStateException` [duplicate]

I tried to let two threads mutually print out something, but the result I get was:

0
Exception in thread "Thread-0" wait in decrement
java.lang.IllegalMonitorStateException
    at java.lang.Object.notify(Native Method)
    at thread.Multithread2.increment(Multithread2.java:38)
    at thread.Multithread2.run(Multithread2.java:18)
    at java.lang.Thread.run(Unknown Source)

I've already included shared object in synchronized block and invoked wait() / notify() on this object. I don't know why it still throws exception.

public class Multithread2 implements Runnable {

    private Integer integer;
    private int method;

    public Multithread2(Integer integer, int method) {
        this.integer = integer;
        this.method = method;
    }

    @Override
    public void run() {
        try {
            Thread.sleep(20);
            for(int i = 0; i < 5; i++) {
                if(method == 1) {
                    increment();
                } else {
                    decrement();
                }
            }
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    public void increment() throws InterruptedException {
        synchronized(integer) {
            while(integer > 0) {
                integer.wait();
            }
            System.out.println(integer);
            integer++;
            integer.notify();
        }
    }

    public void decrement() throws InterruptedException {
        synchronized(integer) {
            while(integer <= 0) {
                integer.wait();
            }
            System.out.println("decrement: " + integer);
            integer--;
            integer.notify();
        }
    }
}

Main method:

Integer integer = new Integer(0);
Thread t1 = new Thread(new Multithread2(integer, 1));
t1.start();
Thread t2 = new Thread(new Multithread2(integer, 2));
t2.start();