JDK 源码解析 —— String

String 类定义：

public final class String
        implements java.io.Serializable, Comparable<String>, CharSequence

/** 底层字符的存储*/
private final char value[];
/** 哈希码*/
private int hash; // Default to 0
private static final long serialVersionUID = -6849794470754667710L;

String类被声明为final的，意味着它不可以被继承。同时内部保持着了一个final 类型的char[]数组。

构造函数

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) {
    this.value = Arrays.copyOfRange(value, offset, offset+count);
}

其他方法

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

public boolean isEmpty() {
    return value.length == 0;
}

public char charAt(int index) {
    if ((index < 0) || (index >= value.length)) {
        throw new StringIndexOutOfBoundsException(index);
    }
    return value[index];
}

public String substring(int beginIndex, int endIndex) {
    int subLen = endIndex - beginIndex;
    if (subLen < 0) {
        throw new StringIndexOutOfBoundsException(subLen);
    }
    return ((beginIndex == 0) && (endIndex == value.length)) ? this
            : new String(value, beginIndex, subLen);
}

// 复制区间:从offset到offset+count处.
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);
        }
        //count==0
        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);
}

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