String s=t.getText().trim();
int l=s.length();
char c=Character.toUpperCase(s.charAt(0));
s=c+s.substring(1);
for(int i=1; i<l; i++)
    {
        if(s.charAt(i)==' ')
        {
            c=Character.toUpperCase(s.charAt(i+1));
            s=s.substring(0, i) + c + s.substring(i+2);
        }
    }
    t.setText(s);

另外，Spring Framework中还有org.springframework.util.StringUtils:

StringUtils.capitalize(str);

使用StringTokenizer类的示例:

String st = "  hello all students";
String st1;
char f;
String fs="";
StringTokenizer a= new StringTokenizer(st);
while(a.hasMoreTokens()){   
        st1=a.nextToken();
        f=Character.toUpperCase(st1.charAt(0));
        fs+=f+ st1.substring(1);
        System.out.println(fs);
} 

我想在接受的答案上添加一个NULL检查和IndexOutOfBoundsException。

String output = input.substring(0, 1).toUpperCase() + input.substring(1);

Java代码:

  class Main {
      public static void main(String[] args) {
        System.out.println("Capitalize first letter ");
        System.out.println("Normal  check #1      : ["+ captializeFirstLetter("one thousand only")+"]");
        System.out.println("Normal  check #2      : ["+ captializeFirstLetter("two hundred")+"]");
        System.out.println("Normal  check #3      : ["+ captializeFirstLetter("twenty")+"]");
        System.out.println("Normal  check #4      : ["+ captializeFirstLetter("seven")+"]");

        System.out.println("Single letter check   : ["+captializeFirstLetter("a")+"]");
        System.out.println("IndexOutOfBound check : ["+ captializeFirstLetter("")+"]");
        System.out.println("Null Check            : ["+ captializeFirstLetter(null)+"]");
      }

      static String captializeFirstLetter(String input){
             if(input!=null && input.length() >0){
                input = input.substring(0, 1).toUpperCase() + input.substring(1);
            }
            return input;
        }
    }

输出:

Normal  check #1      : [One thousand only]
Normal  check #2      : [Two hundred]
Normal  check #3      : [Twenty]
Normal  check #4      : [Seven]
Single letter check   : [A]
IndexOutOfBound check : []
Null Check            : [null]

在这里(希望这能让你明白):

/*************************************************************************
 *  Compilation:  javac Capitalize.java
 *  Execution:    java Capitalize < input.txt
 * 
 *  Read in a sequence of words from standard input and capitalize each
 *  one (make first letter uppercase; make rest lowercase).
 *
 *  % java Capitalize
 *  now is the time for all good 
 *  Now Is The Time For All Good 
 *  to be or not to be that is the question
 *  To Be Or Not To Be That Is The Question 
 *
 *  Remark: replace sequence of whitespace with a single space.
 *
 *************************************************************************/

public class Capitalize {

    public static String capitalize(String s) {
        if (s.length() == 0) return s;
        return s.substring(0, 1).toUpperCase() + s.substring(1).toLowerCase();
    }

    public static void main(String[] args) {
        while (!StdIn.isEmpty()) {
            String line = StdIn.readLine();
            String[] words = line.split("\\s");
            for (String s : words) {
                StdOut.print(capitalize(s) + " ");
            }
            StdOut.println();
        }
    }

}

1. 使用字符串的substring()方法

public static String capitalize(String str) {
    if(str== null || str.isEmpty()) {
        return str;
    }

    return str.substring(0, 1).toUpperCase() + str.substring(1);
}

现在只需调用capitalize()方法将字符串的第一个字母转换为大写字母:

System.out.println(capitalize("stackoverflow")); // Stackoverflow
System.out.println(capitalize("heLLo")); // HeLLo
System.out.println(capitalize(null)); // null

2. Apache Commons Lang

Commons Lang中的StringUtils类提供了capitalize()方法，该方法也可用于此目的:

System.out.println(StringUtils.capitalize("apache commons")); // Apache commons
System.out.println(StringUtils.capitalize("heLLO")); // HeLLO
System.out.println(StringUtils.uncapitalize(null)); // null

将以下依赖项添加到pom.xml文件中(仅适用于Maven):

<dependency>
    <groupId>org.apache.commons</groupId>
    <artifactId>commons-lang3</artifactId>
    <version>3.9</version>
</dependency>

下面是一篇详细解释这两种方法的文章。