字符

小写转大写

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char c1 = 'a';
c1=Character.toUpperCase(c1);

大写转小写

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char c2 = 'B'; 
c2 = Character.toLowerCase(c2);

字符串

全部转换成小写

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String str = "aBcD";
str=str.toLowerCase();

全部转换成大写

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String str = "aBcD";
str=str.toUpperCase();

Java代码

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public static void main(String[] args) {
	char c1 = 'a';
	char c2 = 'B';
	System.out.println("字符大小写转换:");
	System.out.println("转换前:c1="+c1);
	c1=Character.toUpperCase(c1);
	System.out.println("转换后:c1="+c1);
	System.out.println("转换前:c2="+c2);
	c2 = Character.toLowerCase(c2);
	System.out.println("转换后:c2="+c2);
	System.out.println("字符串全部转换成大写或小写");
	String str = "aBcD";
	System.out.println("转换前:" + str);
	System.out.println("转换后:");
	System.out.println("全部转换成小写:"+str.toLowerCase());
	System.out.println("全部转换成大写:"+str.toUpperCase());
	System.out.println("字符串大写变小写,小写变大写:");
	StringBuffer stringBuffer = new StringBuffer(str);
	System.out.println("转换前:"+stringBuffer);
	char[] arr = new char[stringBuffer.length()];
	for (int i = 0; i < arr.length; i++) {
		arr[i] = stringBuffer.charAt(i);
		if (65<=arr[i]&&arr[i]<=90) {
			arr[i] = Character.toLowerCase(arr[i]);
		} else if (97<=arr[i]&&arr[i]<=122) {
			arr[i] = Character.toUpperCase(arr[i]);
		}
	}
	stringBuffer.delete(0, stringBuffer.length());
	for (int i = 0; i < arr.length; i++) {
		stringBuffer.append(arr[i]);
	}
	System.out.println("转换后:"+stringBuffer);
}

运行结果

img