读 String原代码

1.  CharSequence接口定义了一个只读的char序列。String 实现 CharSequence , Serializable , Comaprable<String>

2.  由 char[] value, int offset, int count 和 int hash组成。

3.  构造函数 public String(String original) 基本上来说是没用的，因为String本身是immutable的，没必要copy一下。但有一个用处：节省空间。比如说，你用subString生成的一个String，它是重用原来的char[]的，而原来的那个String你已经不想用了（这个条件很重要）， 你可以用这个构造函数重建一个String来释放空间。

public String(String original) {
        int size = original.count;
        char[] originalValue = original.value;
        char[] v;
        if (originalValue.length > size) {
            // The array representing the String is bigger than the new
            // String itself.  Perhaps this constructor is being called
            // in order to trim the baggage, so make a copy of the array.
            int off = original.offset;
            v = Arrays.copyOfRange(originalValue, off, off+size);
        } else {
            // The array representing the String is the same
            // size as the String, so no point in making a copy.
            v = originalValue;
        }
        this.offset = 0;
        this.count = size;
        this.value = v;
    }

public String(String original) {
        int size = original.count;
        char[] originalValue = original.value;
        char[] v;
        if (originalValue.length > size) {
            // The array representing the String is bigger than the new
            // String itself.  Perhaps this constructor is being called
            // in order to trim the baggage, so make a copy of the array.
            int off = original.offset;
            v = Arrays.copyOfRange(originalValue, off, off+size);
        } else {
            // The array representing the String is the same
            // size as the String, so no point in making a copy.
            v = originalValue;
        }
        this.offset = 0;
        this.count = size;
        this.value = v;
    }