java中的赋值运算符和addAll有什么区别？

Question

I wondered related to a book question in a set chapter?

    Set<String> mySet = new HashSet<>();

    mySet.add("red");
    mySet.add("blue");
    mySet.add("pink");
    mySet.add("black");

    //first presentation

    Set<String> newSetFirst = new HashSet<>();
    newSetSirst.addAll(mySet);

    //second presentation

    Set<String> newSetTwo;
    newSetTwo = mySet;

what difference between first presentation and second presentation?

What mean of newSetSirst.addAll(mySet)?

What mean of newSetTwo = mySet?

Answer 1

First presentation:

Set<String> newSetFirst = new HashSet<>();
newSetFirst.addAll(mySet);

System.out.println(newSetFirst);  // Prints ["red", "blue", "pink", "black"]

mySet.clear();

System.out.println(newSetFirst);  // Prints ["red", "blue", "pink", "black"]

When you addAll , you're just adding everything to an existing set. Updates to the set you added from aren't reflected in that set, because it's entirely separate.

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Second presentation:

Set<String> newSetTwo;
newSetTwo = mySet;

System.out.println(newSetTwo);  // Prints ["red", "blue", "pink", "black"]

mySet.clear();

System.out.println(newSetTwo);  // Prints []

When you assign, you're just giving yourself another variable which points to the same list. mySet == newSetTwo is true: there's only one set. As such, changing mySet changes newSetTwo as well.

Answer 2

First presentation creates a new list and adds every item to it from the first list. If you change one item in the mySetFirst list, the mySet list is not gonna be changed.

The second presentation points to the reference of mySet. If you change one item in newSetTwo, the item is gonna get changed in mySet too. In that case mySet and newSetTwo are the same list.