1- 分层继承2- 可变和不可变的概念3- String3.1- 字符串是一个非常特殊的类3.2- String 字面值 vs. String对象3.3- String的方法3.3.1- length()3.3.2- concat(String)3.3.3- indexOf(..)3.3.4- substring(..)3.3.5- replace3.3.6- 其它实例4- StringBuffer vs StringBuilder

StringBuilder和StringBuffer都是一样的，除了涉及到多线程的使用情况。

至于处理的速度，StringBuilder是最好的，其次是StringBuffer，而最后是String。

考虑下面的一个例子：

// This is a class with value field and name field.

// When you create this class, you cannot reset the value and all other fields from outside.

// This class does not have methods for the purpose of resetting fields from outside.

// It means this class is immutable

public class ImmutableClassExample  {

   private int value;

   private String name;

   public ImmutableClassExample(String name, int value)  {

          this.value = value;

          this.name= name;

   }

   public String getName()  {

          return name;

   }

   public int getValue()  {

         return value;

   }

}

// This is a class owning a value field.

// After creating the object, you can reset values of the value field by calling setNewValue(int) method.

// This is a mutable class.

public class MutableClassExample  {

     private int value;

     public MutableClassExample(int value)  {

           this.value= value;

     }

     public void setNewValue(int newValue)  {

          this.value = newValue;

     }

}

String是不可变的类。String包括各种字段如长度，但在这些字段中的值不能改变。

字符串接受Java的特殊处理，因为它们在程序中经常使用。因此，效率(在计算和存储方面)是至关重要的。

例如：

// Implicit construction via string literal

String str1 = "Java is Hot";

// Explicit construction via new

String str2 = new String("I'm cool");

正如前面提到的，有两种方法来构造字符串：通过指定一个字符串字面量或显式创建通过 new 操作符，并构造一个String对象的隐式构建。 例如，

String s1 = "Hello";              // String literal

String s2 = "Hello";              // String literal

String s3 = s1;                   // same reference

String s4 = new String("Hello");  // String object

String s5 = new String("Hello");  // String object

我们使用以下图片说明来解释它：

可以使用String类的方法equals()方法比较两个字符串的内容。可以使用关系等于运算符'=='来比较两个对象的引用(或指针)。研究以下代码：

String s1 = "Hello";              // String literal

String s2 = "Hello";              // String literal

String s3 = s1;                   // same reference

String s4 = new String("Hello");  // String object

String s5 = new String("Hello");  // String object

s1 == s1;         // true, same pointer

s1 == s2;         // true, s1 and s2 share storage in common pool

s1 == s3;         // true, s3 is assigned same pointer as s1

s1 == s4;         // false, different pointers

s4 == s5;         // false, different pointers in heap

s1.equals(s3);    // true, same contents

s1.equals(s4);    // true, same contents

s4.equals(s5);    // true, same contents

事实上，你应该使用字符串，而是采用了“new”的操作符，这有助于加快程序运行速度。

下面是字符串列表的方法：

• LengthDemo.java

package com.yiibai.tutorial.str;

public class LengthDemo {

    public static void main(String[] args) {

        String str = "This is text";

        int len = str.length();

        System.out.println("String Length is : " + len);

    }

}

运行示例的结果：

• ConcatDemo.java

package com.yiibai.tutorial.str;

public class ConcatDemo {

    public static void main(String[] args) {

        String s1 = "One";

        String s2 = "Two";

        String s3 = "Three";

        // s1.concat(s2) same as s1 + s2

        String s = s1.concat(s2);

        System.out.println("s1.concat(s2) = " + s);

        // s1.concat(s2).concat(s3) same as s1 + s2 + s3;

        s = s1.concat(s2).concat(s3);

        System.out.println("s1.concat(s2).concat(s3) = " + s);

    }

}

运行示例的结果：

s1.concat(s2) = OneTwo

s1.concat(s2).concat(s3) = OneTwoThree

• IndexOfDemo.java

package com.yiibai.tutorial.str;

public class IndexOfDemo {

    public static void main(String[] args) {

        String str = "This is text";

        // Find index within this string of the first occurrence 'i'.

        // ==> 2

        int idx = str.indexOf('i');

        System.out.println("- indexOf('i') = " + idx);

        // Find index within this string of the first occurrence 'i'

        // starting the search at index 4.

        // ==> 5

        idx = str.indexOf('i', 4);

        System.out.println("- indexOf('i',4) = " + idx);

        // index within this string of the first occurrence of "te".

        // ==> 8

        idx = str.indexOf("te");

        System.out.println("- indexOf('te') = " + idx);

    }

}

运行示例的结果：

- indexOf('i') = 2

- indexOf('i',4) = 5

- indexOf('te') = 8

• SubstringDemo.java

package com.yiibai.tutorial.str;

public class SubstringDemo {

    public static void main(String[] args) {

        String str = "This is text";

        // Returns the substring from index 3 to the end of string.

        String substr = str.substring(3);

        System.out.println("- substring(3)=" + substr);

        // Returns the substring from index 2 to index 7.

        substr = str.substring(2, 7);

        System.out.println("- substring(2, 7) =" + substr);

    }

}

运行示例的结果：

- substring(3)=s is text

- substring(2, 7) =is is

与更换的一些方法。

// Returns a new string resulting from replacing all occurrences of

// oldChar in this string with newChar.

public String replace(char oldChar, char newChar)

// Replaces each substring of this string that matches the given

// 'regular expression' with the given replacement.

public String replaceAll(String regex, String replacement)

// Replaces the first substring of this string that matches

// the given <'regular expression' with the given replacement.

public String replaceFirst(String regex, String replacement)

• ReplaceDemo.java

package com.yiibai.tutorial.str;

public class ReplaceDemo {

    public static void main(String[] args) {

        String str = "This is text";

        // Replace the character 'i' by 'x'.

        String s2 = str.replace('i', 'x');

        System.out.println("- s2=" + s2);

        // Replace all the strings match "is" by "abc". (Regular Expression)

        String s3 = str.replaceAll("is", "abc");

        System.out.println("- s3=" + s3);

        // Replaces the first substring of this string that matches "is" by "abc".

        String s4 = str.replaceFirst("is", "abc");

        System.out.println("- s4=" + s4);

        // (See also document the regular expression)

        // Replace all substring matching expression:

        // "is|te": means "is" or "te" replaced by "+".

        String s5 = str.replaceAll("is|te", "+");

        System.out.println("- s5=" + s5);

    }

}

运行示例的结果：

- s2=Thxs xs text

- s3=Thabc abc text

- s4=Thabc is text

- s5=Th+ + +xt

• StringOtherDemo.java

package com.yiibai.tutorial.str;

public class StringOtherDemo {

    public static void main(String[] args) {

        String str = "This is text";

        System.out.println("- str=" + str);

        // Return lower case string.

        String s2 = str.toLowerCase();

        System.out.println("- s2=" + s2);

        // Return upper case string

        String s3 = str.toUpperCase();

        System.out.println("- s3=" + s3);

        // Check string started by "This" or not.

        boolean swith = str.startsWith("This");

        System.out.println("- 'str' startsWith This ? " + swith);

        // A string with whitespace in beginning and end.

        // Note: \t is tab character

        // \n is new line character

        str = " \t Java is hot!  \t \n ";

        System.out.println("- str=" + str);

        // Returns a copy of the string, with leading and trailing whitespace omitted.

        String s4 = str.trim();

        System.out.println("- s4=" + s4);

    }

}

运行示例的结果：

- str=This is text

- s2=this is text

- s3=THIS IS TEXT

- 'str' startsWith This ? true

- str= 	 Java is hot!  	 

- s4=Java is hot!

StringBuffer的方法(StringBuilder相似)

// Constructors

StringBuffer()             // an initially-empty StringBuffer

StringBuffer(int size)     // with the specified initial size

StringBuffer(String s)     // with the specified initial content

// Length

int length()

// Methods for building up the content

// type could be primitives, char[], String, StringBuffer, etc

StringBuffer append(type arg)  // ==> note above!

StringBuffer insert(int offset, type arg) // ==> note above!

// Methods for manipulating the content

StringBuffer delete(int start, int end)

StringBuffer deleteCharAt(int index)

void setLength(int newSize)

void setCharAt(int index, char newChar)

StringBuffer replace(int start, int end, String s)

StringBuffer reverse()

// Methods for extracting whole/part of the content

char charAt(int index)

String substring(int start)

String substring(int start, int end)

String toString()

// Methods for searching

int indexOf(String searchKey)

int indexOf(String searchKey, int fromIndex)

int lastIndexOf(String searchKey)

int lastIndexOf(String searchKey, int fromIndex)

• StringBuilderDemo.java

package com.yiibai.tutorial.strbb;

public class StringBuilderDemo {

    public static void main(String[] args) {

        // Create StringBuilder object

        // with no characters in it and

        // an initial capacity specified by the capacity argument

        StringBuilder sb = new StringBuilder(10);

        // Append the string Hello ... on sb.

        sb.append("Hello...");

        System.out.println("- sb after appends a string: " + sb);

        // append a character

        char c = '!';

        sb.append(c);

        System.out.println("- sb after appending a char: " + sb);

        // Insert a string at index 5

        sb.insert(8, " Java");

        System.out.println("- sb after insert string: " + sb);

        // Delete substring at index 5 to 8

        sb.delete(5,8);

        System.out.println("- sb after delete: " + sb);

    }

}

运行示例的结果：

- sb after appends a string: Hello...

- sb after appending a char: Hello...!

- sb after insert string: Hello... Java!

- sb after delete: Hello Java!