can somebody tell me what the error in this code is please?

Code:
// NOTE:
// client.sSpaceRep = "%20";

	public String EncodeChatMsg(String ChatMsgIn){
		String sReplace;
		// Start the process of editing the message
		sReplace = ChatMsgIn;
		// Find any double spaces
		while(sReplace.indexOf("  ") != -1){
			// change the double to single space
			sReplace.Replace("  "," ");
		}
		// replace single spaces with %20
		sReplace.Replace(" ",client.sSpaceRep);
		// Return the edited msg
		return sReplace;		
	}

Code:
C:\Programming\TheWalks\TheWalksChat\JavaChat\chatclient\ClientInterface.java:253: cannot resolve symbol
symbol  : method Replace (java.lang.String,java.lang.String)
location: class java.lang.String
			sReplace.Replace("  "," ");
                                ^
C:\Programming\TheWalks\TheWalksChat\JavaChat\chatclient\ClientInterface.java:255: cannot resolve symbol
symbol  : method Replace (java.lang.String,java.lang.String)
location: class java.lang.String
		sReplace.Replace(" ",client.sSpaceRep);
                        ^