Java String源码笔记

一、类定义

public final class String
implements java.io.Serializable, Comparable<String>, CharSequence{
    /** The value is used for character storage. */
    private final char value[];

    /** Cache the hash code for the string */
    private int hash; // Default to 0

    ...
}

1、实现Serializable，可以实现序列化。序列化是指将对象的状态信息转换为可以存储或传输的形式的过程。

2、使用final表示不能被继承，里面的方法不能被重写，使用char数组来存储字符串元素。

3、hash储存字符串的hash码，首次调用 hashCode() 方法时，会计算并存储哈希码值，之后再次调用 hashCode() 时直接返回缓存的值，避免重复计算，提高性能

二、构造函数

public String() {
    this.value = "".value;
}
 
public String(String original) {
this.value = original.value;
this.hash = original.hash;
}
 
public String(char value[]) {
    this.value = Arrays.copyOf(value, value.length);
}
 
public String(char value[], int offset, int count) {
    if (offset < 0) {
        throw new StringIndexOutOfBoundsException(offset);
    }
    if (count <= 0) {
        if (count < 0) {
            throw new StringIndexOutOfBoundsException(count);
        }
        if (offset <= value.length) {
            this.value = "".value;
            return;
        }
    }
    // Note: offset or count might be near -1>>>1.
    if (offset > value.length - count) {
        throw new StringIndexOutOfBoundsException(offset + count);
    }
    this.value = Arrays.copyOfRange(value, offset, offset+count);
}
 
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;
}
 
 
@Deprecated
public String(byte ascii[], int hibyte, int offset, int count) {
    checkBounds(ascii, offset, count);
    char value[] = new char[count];
 
    if (hibyte == 0) {
        for (int i = count; i-- > 0;) {
            value[i] = (char)(ascii[i + offset] & 0xff);
        }
    } else {
        hibyte <<= 8;
        for (int i = count; i-- > 0;) {
            value[i] = (char)(hibyte | (ascii[i + offset] & 0xff));
        }
    }
    this.value = value;
}
 
@Deprecated
public String(byte ascii[], int hibyte) {
    this(ascii, hibyte, 0, ascii.length);
}
 
public String(byte bytes[], int offset, int length, String charsetName)
            throws UnsupportedEncodingException {
        if (charsetName == null)
            throw new NullPointerException("charsetName");
        checkBounds(bytes, offset, length);
        this.value = StringCoding.decode(charsetName, bytes, offset, length);
    }
 
 
public String(byte bytes[], int offset, int length, Charset charset) {
    if (charset == null)
        throw new NullPointerException("charset");
    checkBounds(bytes, offset, length);
    this.value =  StringCoding.decode(charset, bytes, offset, length);
}
 
public String(byte bytes[], String charsetName)
        throws UnsupportedEncodingException {
    this(bytes, 0, bytes.length, charsetName);
}
 
public String(byte bytes[], Charset charset) {
    this(bytes, 0, bytes.length, charset);
}
 
public String(byte bytes[], int offset, int length) {
    checkBounds(bytes, offset, length);
    this.value = StringCoding.decode(bytes, offset, length);
}
 
public String(byte bytes[]) {
    this(bytes, 0, bytes.length);
}
 
public String(StringBuffer buffer) {
    synchronized(buffer) {
        this.value = Arrays.copyOf(buffer.getValue(), buffer.length());
    }
}
 
public String(StringBuilder builder) {
    this.value = Arrays.copyOf(builder.getValue(), builder.length());
}
 
String(char[] value, boolean share) {
    // assert share : "unshared not supported";
    this.value = value;
}