java - 如何看懂java String源码

Question

它是如何工作的？我不明白它是如何发展到这样的地步的，每当你改变原始字符串中的某些内容时，每次都会创建一个新字符串。 offset、value、count分别代表什么？

private final char value[];  
   private final int offset;  
   private final int count;  

public String() {  
       this.offset = 0;  
       this.count = 0;  
       this.value = new char[0];  
   }  

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(char value[]) {  
       int size = value.length;  
       this.offset = 0;  
       this.count = size;  
       this.value = Arrays.copyOf(value, size);  
   }  

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

public String substring(int beginIndex, int endIndex) {  
       if (beginIndex < 0) {  
           throw new StringIndexOutOfBoundsException(beginIndex);  
       }  
       if (endIndex > count) {  
           throw new StringIndexOutOfBoundsException(endIndex);  
       }  
       if (beginIndex > endIndex) {  
           throw new StringIndexOutOfBoundsException(endIndex - beginIndex);  
       }  
       return ((beginIndex == 0) && (endIndex == count)) ? this :  
           new String(offset + beginIndex, endIndex - beginIndex, value);         
   }  

public String concat(String str) {  
       int otherLen = str.length();  
       if (otherLen == 0) {  
           return this;  
       }  
       char buf[] = new char[count + otherLen];  
       getChars(0, count, buf, 0);  
       str.getChars(0, otherLen, buf, count);  
       return new String(0, count + otherLen, buf);       
   }  

public static String valueOf(char data[]) {  
       return new String(data);       
   }  

public static String valueOf(char c) {  
       char data[] = {c};  
       return new String(0, 1, data);       
   }  

Answer 1

该值是字符串的底层字符数组。偏移量是字符串开始的位置，计数是它的长度。一个字符串可能在数组 {'a','b','c','d','e'} 中，计数为 3，偏移量为 1，它是 "bcd"。这样就不会为每个子字符串操作复制数组。