We need spaces between arguments in commandline because the commandline need to know which is the first argument, which is the second and so on. However when we use ProcessBuilder, we can pass an array to it, so we do not need to add those spaces to differentiate the arguments. The ProcessBuilder will directly pass the command array to the exec after some checking. For example,

private static final String JAVA_CMD = "java";
private static final String CP = "-cp";

private static final String CLASS_PATH = "../bin";
private static final String PROG = "yr12.m07.b.Test";
private static final String[] CMD_ARRAY = { JAVA_CMD, CP, CLASS_PATH, PROG };
ProcessBuilder processBuilder = new ProcessBuilder(CMD_ARRAY);

The above code will work perfectly.

Moreover, you can use

Runtime.getRuntime().exec("java -cp C:/testt Test");

But it is more convenient to use ProcessBuilder, one reason is that if our argument contains space we need to pass quote in Runtime.getRuntime().exec() like java -cp C:/testt \"argument with space\", but with ProcessBuilder we can get rid of it.

ProcessBuilder processBuilder = new ProcessBuilder("command", "The first argument", "TheSecondWithoutSpace");