JDK源码分析之String、StringBuilder和StringBuffer

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

/*字符数组value，存储String中实际字符 */
private final char value[];
/*字符串的哈希值 默认值0*/
private int hash; 
/*字符串的哈希值 默认值0*/
/*一个比较器，用来排序String对象, compareToIgnoreCase方法中有使用 */
public static final Comparator<String> CASE_INSENSITIVE_ORDER
      = new CaseInsensitiveComparator();

//String str = new String(“123”)
public String(String original) {
  this.value = original.value;
  this.hash = original.hash;
}

//String str3 = new String(new char[] {'1','2','3'});
public String(char value[]) {
   //将字符数组值copy至value
  this.value = Arrays.copyOf(value, value.length); 
 }

public boolean equals(Object anObject) {
  //直接将对象引用相比较，相同返回true
  if (this == anObject) {
   return true;
  }
  //比较当前对象与anObject的字符序列value
  if (anObject instanceof String) {
   String anotherString = (String)anObject;
   int n = value.length;
   if (n == anotherString.value.length) {
    char v1[] = value;
    char v2[] = anotherString.value;
    int i = 0;
    while (n-- != 0) {
     if (v1[i] != v2[i])
      return false;
     i++;
    }
    return true;
   }
  }
  return false;
 }

public int compareTo(String anotherString) {
  int len1 = value.length;
  int len2 = anotherString.value.length;
  //取长度较小的字符串的长度
  int lim = Math.min(len1, len2);
  char v1[] = value;
  char v2[] = anotherString.value;

  int k = 0;
  while (k < lim) {
   //将两个字符串的字符序列value逐个比较，如果不等，则返回该位置两个字符的Unicode 之差
   char c1 = v1[k];
   char c2 = v2[k];
   if (c1 != c2) {
    return c1 - c2; //返回Unicode 之差

   }
   k++;
  }
  //长度较小的字符串所有位都比较完，则返回两个字符串长度之差
  //如果两个字符串相同，那么长度之差为0，即相同字符串返回0
  return len1 - len2;
 }

public int compare(String s1, String s2) {
   int n1 = s1.length();
   int n2 = s2.length();
   int min = Math.min(n1, n2);
   for (int i = 0; i < min; i++) {
    char c1 = s1.charAt(i);
    char c2 = s2.charAt(i);
    if (c1 != c2) {
     c1 = Character.toUpperCase(c1);
     c2 = Character.toUpperCase(c2);
     if (c1 != c2) {
      c1 = Character.toLowerCase(c1);
      c2 = Character.toLowerCase(c2);
      if (c1 != c2) {
       // No overflow because of numeric promotion
       return c1 - c2;
      }
     }
    }
   }
   return n1 - n2;
  }

String str1 = new String("123");
  String str2 = "123";
  String str3 = "123";
  String str4 = str1.intern();
  System.out.println(str1==str2); // false str1在堆中创建对象，str2在常量池中创建对象
  System.out.println(str2==str3); // true str2在常量池中创建对象,str3直接返回的str2创建的对象的引用 所以str2和str3指向常量池中同一个对象
  System.out.println(str4==str3); // true str4返回常量池中值为"123"的对象，因此str4和str2、str3都相等

public class StringTest {
 public static final String X = "ABC"; // 常量X
 @Test
 public void Test() {

  String str5 = new String("ABC");
  String str6 = str5+"DEF"; //堆中创建
  String str7 = "ABC"+"DEF"; //常量池
  String str8 = X+"DEF";  //X为常量,值是固定的,因此X+"DEF"值已经定下来为ABCDEF，实际上编译后得代码相当于String str8 = "ABCDEF"
  String str9 = "ABC";  
  String str10 = str9+"DEF"; //堆中
  
  System.out.println(str6==str7); //false
  System.out.println(str8==str7); //true
  System.out.println(str10==str7); //false

  System.out.println(X==str9); //true
} }

/** The value is used for character storage. */
private final char value[];

String str1="123";
   String str2="456";
   String str3="789";   
   String str4 = "123" + "456" + "789"; //常量相加，编译器自动识别 String str4=“123456789”   
   String str5 = str1 + str2 + str3; //字符串变量拼接，推荐使用StringBuilder   
   StringBuilder sb = new StringBuilder(); 
   sb.append(str1);
   sb.append(str2);
   sb.append(str3);

public final class StringBuilder
 extends AbstractStringBuilder
 implements java.io.Serializable, CharSequence
{

 //拼接字符串
 @Override
 public StringBuilder append(String str) {
 //调用父类AbstractStringBuilder.append super.append(str); return this; } }

abstract class AbstractStringBuilder implements Appendable, CharSequence {
 /**
  * 存储字符串的字符数组，非final类型，区别于String类
  */
 char[] value;

 /**
  * The count is the number of characters used.
  */
 int count;

 public AbstractStringBuilder append(String str) {
  if (str == null)
   return appendNull();
  int len = str.length();
   //检查是否需要扩容
  ensureCapacityInternal(count + len);
  //字符串str拷贝至value  
  str.getChars(0, len, value, count);
  count += len;
  return this;
}


 private void ensureCapacityInternal(int minimumCapacity) {
  // overflow-conscious code
  // minimumCapacity=count+str.length
  //拼接上str后的容量 如果 大于value容量，则扩容
  if (minimumCapacity - value.length > 0) {
    
    //扩容，并将当前value值拷贝至扩容后的字符数组，返回新数组引用
   value = Arrays.copyOf(value,
     newCapacity(minimumCapacity));
  }
 }

 //StringBuilder扩容
 private int newCapacity(int minCapacity) {
  // overflow-conscious code
  // 计算扩容容量
  // 默认扩容后的数组长度是按原数（value[]）组长度的2倍再加上2的规则来扩展,为什么加2？
  int newCapacity = (value.length << 1) + 2;
  if (newCapacity - minCapacity < 0) {
   newCapacity = minCapacity;
  }
  return (newCapacity <= 0 || MAX_ARRAY_SIZE - newCapacity < 0)
   ? hugeCapacity(minCapacity)
   : newCapacity;
 }
}