/**
 * The {@code String} class represents character strings. All
 * string literals in Java programs, such as {@code "abc"}, are
 * implemented as instances of this class.
 * <p>
 * Strings are constant; their values cannot be changed after they
 * are created. String buffers support mutable strings.
 * Because String objects are immutable they can be shared. For example:
 * ...
 */

public final class String { 
 private final char value[];
}

/**
 * 定義一個字符串
 */
String str1 = "helloworld";
String str2 = "helloworld";
//也可以,但基本不這樣寫
String str3 = new String("helloworld");
System.out.println(str1 == str2);
System.out.println(str1 == str3);

//運行結(jié)果 true, false

System.out.println(str1 == str3.intern());
//運行結(jié)果為true

/** 
 * Returns a canonical representation for the string object. 
 * <p> 
 * A pool of strings, initially empty, is maintained privately by the 
 * class {@code String}. 
 * <p> 
 * When the intern method is invoked, if the pool already contains a 
 * string equal to this {@code String} object as determined by 
 * the {@link #equals(Object)} method, then the string from the pool is 
 * returned. Otherwise, this {@code String} object is added to the 
 * pool and a reference to this {@code String} object is returned. 
 * <p> 
 * It follows that for any two strings {@code s} and {@code t}, 
 * {@code s.intern() == t.intern()} is {@code true} 
 * if and only if {@code s.equals(t)} is {@code true}. 
 * <p> 
 * All literal strings and string-valued constant expressions are 
 * interned. String literals are defined in section 3.10.5 of the 
 * <cite>The Java&trade; Language Specification</cite>. 
 * 
 * @return a string that has the same contents as this string, but is 
 *  guaranteed to be from a pool of unique strings. 
 */
 public native String intern();

String str4 = new String("helloChina");
System.out.println(str4.intern() == str4);//false
str4 = str4.intern();
String str5 = "helloChina";
String str6 = "helloZhonghua"
System.out.println(str4 == str5);//true

str6 = "helloHuaxia";

String str7 = "good good" + " study";
String str8 = "good good study";
system.out.println(str7 == str8);

String str7 = "good good study";
String str8 = "good good study";

String str9 = "good good ";
String str10 = str9 + "study";
system.out.println(str8 == str10);//false

//str1 == "hello,world ";

//獲取長度
str1.length()//12;

//截取位置2到5之間的字符串（包括位置2，不包括位置5，從0開始）
str1.substring(2,5);//"llo"

//判斷是否含有字符串“ello”
str1.contains("ello");//true,通過indexOf實現(xiàn)

//獲取ello在str1中的開始位置
str1.indexOf("ello");//1

//將字符串轉(zhuǎn)化為字符串?dāng)?shù)據(jù)
str1.split(",");//["hello","world"]

//去掉字符串兩側(cè)空格
str1.trim();//"hello,world"