欢迎使用CSDN-markdown编辑器

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一、String 与 StringBuilder

String不可变字符串，而StringBuilder可变，考虑效率问题少量字符串连接用String大量字符串连接用StringBuilder（append()方法）例子：

    StringBuilder builder = new StringBuilder();            builder.append("Ich");            builder.append(" ");            builder.append("liebe");            builder.append(" ");            builder.append("dich");            builder.append("!");            System.out.println(builder);

二、子字符串
subString() 方法 [ a, b ), 从0开始， b-a为字符串长度。
split()方法提取字符串
例子：

     System.out.println(builder.substring(0,3));            System.out.println(builder.            toString().split(" ")[0]);

**三、字符串比较**equals() 方法 ,***警告：不能用"=="，其比较的是对象*****四、字符串与数字互转** 数字转字符串  toString()方法 字符串转数字  Integer.parseInt() 或 Double.parseDouble()**五、String类API**注意：String类是不可改变的，故任何一个String方法都不可改变自己的字符串！见总代码如下

    StringBuilder builder = new StringBuilder();            builder.append("Ich");            builder.append(" ");            builder.append("liebe");            builder.append(" ");            builder.append("dich");            builder.append("!");            System.out.println(builder);            System.out.println(builder.substring(0, 3));            System.out.println(builder.                toString().split(" ")[0]);            System.out.println(builder.                substring(0, 3) == "Ich");            System.out.println(builder.                substring(0, 3).equals("Ich"));            System.out.println("\nstart and end");            System.out.println(builder.                toString().startsWith("I"));            System.out.println(builder.                toString().startsWith("i"));            System.out.println(builder.toString().                endsWith("!"));            System.out.println(builder.toString().                endsWith(" "));            System.out.println("\ncontain");            System.out.println(builder.toString().                contains("liebe"));            System.out.println(builder.toString().                contains("love"));            System.out.println("\nindexOf");            System.out.println(builder.indexOf("Ich"));            System.out.println(builder.indexOf("I", 5));            System.out.println("\nlastIndexOf");            System.out.println(builder.lastIndexOf("h"));            System.out.println("\replace");            builder.replace(builder.indexOf("liebe") + 5,              builder.lastIndexOf("liebe") + 6 , " nicht");            System.out.println(builder);            System.out.println("\ntoUpperCase");            builder.toString().toUpperCase();            System.out.println(builder);            System.out.println(builder.                toString().toUpperCase());            System.out.println("\ntoLowCase");            builder.toString().toLowerCase();            System.out.println(builder);            System.out.println(builder.                toString().toLowerCase());            System.out.println("\ntrim");            builder = new StringBuilder(" ").                append(builder);            builder.toString().trim();            System.out.println(builder);            System.out.println(builder.toString().trim());

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