Java:thinging in java p158 exercise 1

Create a class with a String field that is initialized at the point of definition, and another one that is initialized by the constructor. What is the difference between the two approaches.

class Test{
    String s1;
    String s2="hello world";
    String s3;
    Test(){
        s3="hello java";
    }
}
public class ConstructorTest {

    public static void main(String[]args){
        
        Test t=new Test();
        System.out.println(t.s1);
        System.out.println(t.s2);
        System.out.println(t.s3);
        
    }
}

输出

null
hello world
hello java