package section.five;

/**
 * Created by mac on 2017/7/16.
 */
public class ExerciseTwo {
    String str1 = "hello";
    String str2 = null;
    String str3 = str2;

    public ExerciseTwo(String str2){
            this.str2 = str2;
    }
}

package section.five;

/**
 * Created by mac on 2017/7/16.
 */
public class Main {
    public static void main(String[] args){
        ExerciseTwo exerciseTwo = new ExerciseTwo("world");

        System.out.println("str1: "+exerciseTwo.str1);

        System.out.println("str2: "+exerciseTwo.str2);
        System.out.println("str3: "+exerciseTwo.str3);
    }
}
 

輸出結果:

str1: hello
str2: world
str3: null

輸出結果可得:

在定義時就被初始化了的String域,在物件被例項化時引用str1 被賦值一次

另一個通過構造器初始化的String域,在物件被例項化時引用str2 被賦值兩次,第一次賦值 null,在賦值worle