java-substring

String x = "abcdef";
x = x.substring(1,3);
System.out.println(x);//输出bc

来看jdk6中String的substring

//JDK 6
//该构造器非公开
String(int offset, int count, char value[]) {
    this.value = value;
    this.offset = offset;
    this.count = count;
}
 
public String substring(int beginIndex, int endIndex) {
    //check boundary
    return  new String(offset + beginIndex, endIndex - beginIndex, value);
}

 jdk6中substring确实新创建了一个String对象，但问题在于该对象的char[] value字段仍然是旧String对象的（新瓶装旧酒），设想如果就string很长，新string很短，这就造成了内存泄漏。

一种解决方法

x = x.substring(x, y) + ""//最终会创建新的char[]

再来看jdk7的substring

//JDK 7
//其实在jdk6中就有该构造器，只不过substring未调用
public String(char value[], int offset, int count) {
    //check boundary
    this.value = Arrays.copyOfRange(value, offset, offset + count);//一份全新的拷贝
}
 
public String substring(int beginIndex, int endIndex) {
    //check boundary
    int subLen = endIndex - beginIndex;
    return new String(value, beginIndex, subLen);
}

参考：https://www.programcreek.com/2013/09/the-substring-method-in-jdk-6-and-jdk-7/