如何在Java中使用字符串处理函数实现常见操作

String str = "  Hello World  ";
str = str.trim();
if (str.isEmpty()) {
    System.out.println("字符串为空");
} else {
    System.out.println("字符串不为空");
}

String str = "Hello World";
String subStr = "World";
if (str.contains(subStr)) {
    System.out.println("字符串包含子字符串");
} else {
    System.out.println("字符串不包含子字符串");
}

String str = "Hello World";
String newStr = str.replace("World", "Java");
System.out.println(newStr);

String str = "apple,banana,grape";
String[] fruits = str.split(",");
for (String fruit : fruits) {
    System.out.println(fruit);
}

String str1 = "Hello";
String str2 = "World";
String str3 = str1.concat(str2);
System.out.println(str3);

String str = "Hello World";
String lowerStr = str.toLowerCase();
String upperStr = str.toUpperCase();
System.out.println(lowerStr);
System.out.println(upperStr);

String str = "Hello World";
int len = str.length();
System.out.println("字符串长度为：" + len);