字符串替换空格

题目：请实现一个函数，把字符串中的每个空格替换成"%20"，例如“We are happy.”，则输出“We%20are%20happy.”。

public class Test3 {
    /**
     * 题目：请实现一个函数，把字符串中的每个空格替换成"%20"，
     * 例如“We are happy.”，则输出“We%20are%20happy.”。
     */
    /**
     * 将空格替换成对应成字符串
     *
     * @param originalStr 原字符串
     * @param replaceStr 替换字符串
     * @return
     */
    private static String replaceChar(String originalStr, String replaceStr) {
        // 对传入参数做为空判断
        if(originalStr == null){
            return null;
        }
        StringBuilder sb = new StringBuilder();
        int length = originalStr.length();
        for (int i = 0; i < length; i++) {
            if(originalStr.charAt(i) == ' '){
             sb.append(replaceStr);   
            }
            sb.append(originalStr.charAt(i));
        }
        return sb.toString();
    }

    public static void main(String[] args) {
        System.out.println(replaceChar(" ","%20"));
        System.out.println(replaceChar("We are happy.","%20"));
        System.out.println(replaceChar(" W e a re happy.","%20"));
    }
}