小白求问getter和setter问题

package com.imooc;

public class Demo1 {

   private int num1;

public int getNum1(){

return num1;

}

public void setNum1(int newNum1){

num1=newNum1;

}

以上是对的程序，通过下面程序调用

package com.imooc;

public class Demo2 {

public static void main(String[] args) {

Demo1 hello3=new Demo1();

hello3.setNum1(55);

System.out.println(hello3.getNum1());

}    

}

再看下面

public int setNum1(int newNum1){

num1=newNum1;

return num1;

}

这种写法没提示出错，但是调用不了是为什么，下面是上面的调用程序

package com.imooc;

public class Demo2 {

public static void main(String[] args) {

Demo1 hello3=new Demo1();

hello3.setNum1(55);

System.out.println(hello3.setNum1());

}    

}在System.out.println(hello3.setNum1());的setNum1()下面出错了，是怎么回事