String源码(1.8)

1.String  存储的值就是一个char数组

 /** The value is used for character storage. */ 2 private final char value[]; 

2.传入int作为参数，这个int是这个字对应的Unicode(16进制数)。每个最大65535 0xFFFF

public static final int MIN_CODE_POINT = 0x000000;

public static final int MAX_CODE_POINT = 0X10FFFF;

UTF-16中的基本单位是两个字节的码元,基本的码元范围是(0x0000-0xFFFF), UTF-16的字符映射范围是(U+0000,U+10FFFF),

当一个生僻字符需要使用0xFFFF以上的映射范围时,其需要使用两个码元(4Byte)进行表示. 其映射规则如下

第一个码元(前导代理)范围:0xD800 - 0xDBFF

第二个码元(后尾代理)范围:0xDC00 - 0xDFFF

有:(0xDBFF-0xD800+1)*(0xDFFF-0xDC00+1) === (0x10FFFF-0xFFFF)双射

所以(0xD800 - 0xDBFF)范围内的码元不能单独表示字符,其必须与后尾代理一起构成一个完整字符.

 参考：https://www.joelonsoftware.com/2003/10/08/the-absolute-minimum-every-software-developer-absolutely-positively-must-know-about-unicode-and-character-sets-no-excuses/

    public String(int[] codePoints, int offset, int count) {
        if (offset < 0) {
            throw new StringIndexOutOfBoundsException(offset);
        }
        if (count <= 0) {
            if (count < 0) {
                throw new StringIndexOutOfBoundsException(count);
            }
            if (offset <= codePoints.length) {
                this.value = "".value;
                return;
            }
        }
        // Note: offset or count might be near -1>>>1.
        if (offset > codePoints.length - count) {
            throw new StringIndexOutOfBoundsException(offset + count);
        }

        final int end = offset + count;

        // Pass 1: Compute precise size of char[]
        int n = count;
        for (int i = offset; i < end; i++) {
            int c = codePoints[i];
            if (Character.isBmpCodePoint(c))
                continue;
            else if (Character.isValidCodePoint(c))
                n++;
            else throw new IllegalArgumentException(Integer.toString(c));
        }

        // Pass 2: Allocate and fill in char[]
        final char[] v = new char[n];

        for (int i = offset, j = 0; i < end; i++, j++) {
            int c = codePoints[i];
            if (Character.isBmpCodePoint(c))
                v[j] = (char)c;
            else
                Character.toSurrogates(c, v, j++);
        }

        this.value = v;
    }

Character.isBmpCodePoint(c) 判断是不是只有一个码元的字符，
Character.isValidCodePoint(c) 判断在字符范围内。此时n++，这个int要用2个char表示。

Character.toSurrogates(c, v, j++) 将int分解成2个char

3.length()返回的是码元char的数量，而不是字的数量，有些字要占两个char

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

4.String.join  免去StringBuild自己拼还要去掉最后一个delimiter

public static String join(CharSequence delimiter, CharSequence... elements)

5.native 关键字 调用别的语言的代码。

public native String intern();

深入解析String#intern

https://tech.meituan.com/in_depth_understanding_string_intern.html