In Java 1.4 onwards the function String.replaceAll does this.
For older versions of Java, you can use this instead:
public static String replaceSubString(String source, String pattern, String replace)
{
if (source!=null)
{
final int len = pattern.length();
StringBuffer sb = new StringBuffer();
int found = -1;
int start = 0;
while( (found = source.indexOf(pattern, start) ) != -1) {
sb.append(source.substring(start, found));
sb.append(replace);
start = found + len;
}
sb.append(source.substring(start));
return sb.toString();
}
else return "";
}