方法一：

public class Demo7 {
	public static void main(String[] args) {
		StringBuffer str = new StringBuffer();
		str.append("We are family");
		System.out.println(replaceSpace(str));
	}
	public static String replaceSpace(StringBuffer str) {
		String str1=str.toString();
		String str2 = str1.replace(" ", "%20");
		return str2;
	}
}
 

方法二：

public class Demo8 {
	public static void main(String[] args) {
		System.out.println(replaceSpace(new StringBuffer("We are family")));
	}
	public static String replaceSpace(StringBuffer str) {
		String str1=str.toString();
		char[] ch = str1.toCharArray();
		StringBuilder sBuilder = new StringBuilder();
		for (char c : ch) {
			if (c==' ') {
				sBuilder.append("%20");
			}else {
				sBuilder.append(c);
				}
		}
		String str2 = sBuilder.toString();
		return str2;
	}
}