public void over(File src,File dest) throws IOException{
        Properties srcP= new Properties();
        InputStream in = new FileInputStream(src);
        srcP.load(in);
        in.close();
        
        Properties destP = new Properties();
        in = new FileInputStream(dest);
        destP.load(in);
        in.close();
        
        Set destKeys = destP.keySet();
        Set srcKeys = srcP.keySet();
        
        for(Object key:srcKeys){
            if(!destKeys.contains(key)){
                //不包含
                destP.put(key, srcP.get(key));
            }
        }
        
        OutputStream out = new FileOutputStream(dest);
        destP.store(out, "By shadao");
        out.close();
    }

------解决方案--------------------
楼上的正解！
可以用!destP.contains(key) 方法来判断是否包含key