Java - 计算字符串长度

Question

While initializing array, Java ask length for that array. Otherwise we get compile-time error.

char[] b = new char[3];

Here is the length of that array .we can get it from

When we see constructor of String class

 public String(String original) {
    this.value = original.value;
    this.hash = original.hash;
}

Where that original parameter in String constructor came? How that get its length?

In String length method

    public int length() {
    return value.length;
}

MyQuestion #1

How can we get the length without initialize array(Since no array initialization inside constructor)?

MyQuestion #2

String s = new String("MyString");

In the above line I am creating instance for String class, when I checked the constructor for the String class I come across the following code for the constructor

 public String(String original) {
    this.value = original.value;
    this.hash = original.hash;
}

My doubt is inside the constructor how they are having String class itself as a parameter? From where it is loading?

Answer 1

The following statement constructs a String instance s out of a "String literal" in this case "MyString"

String s = new String("MyString");

It is equivalent and less preferable to

String s = "MyString";

According to the official docs

1. A string literal is a reference to an instance of class String (§4.3.1, §4.3.3).
2. A string literal always refers to the same instance of class String (interning)

Regarding specifically the length of "MyString" , the compiler would count the number of chars in the literal when parsing the source code and initialize the char[] inside that string with the appropriate length. Then it would create a new String s with the same length

As a side note, you can (for similar reasons) create an array without specifying the length yourself. The compiler will count the number of elements and set the length accordingly Eg

char[] b = {'a', 'b', 'c'};

Answer 2

Java allows to give parameter of same class object in which you are writing constructor or other methods.

 public String(String original) {
    this.value = original.value;
    this.hash = original.hash;
}

String class gets length from hash, as hash has a unique code which tells how many chracters are there.