How to convert from String to a primitive type or standard java Wrapper types

I have a java.lang.reflect.InvocationHandler and I need to implement the method invoke()

I have a value of type java.lang.String from my elaboration and I need to convert this value to the appropriate returnType expected by the method (it can be a primitive like int, boolean, double or wrapper classes like Boolean, Integer, Double, Float, etc).

Example:

public Object invoke(Object proxy, Method method, Object[] args) 
        throws Throwable {
    String computedValue = compute(...);
    return convert(method.getReturnType(), computedValue);
}

private Object convert(Class<?> returnType, String stringValue) {
    return ...; // what's the simplest way?
}

I am not expecting to simply implement an automatic conversion between complex objects, but I expect a simple way to convert from String to the standard java types.

I've seen (too) many times stuff like this, but it doesn't seem appropriate to me:

public static Object toObject( Class clazz, String value ) {
    if( Boolean.class.isAssignableFrom( clazz ) ) return Boolean.parseBoolean( value );
    if( Byte.class.isAssignableFrom( clazz ) ) return Byte.parseByte( value );
    if( Short.class.isAssignableFrom( clazz ) ) return Short.parseShort( value );
    if( Integer.class.isAssignableFrom( clazz ) ) return Integer.parseInteger( value );
    if( Long.class.isAssignableFrom( clazz ) ) return Long.parseLong( value );
    if( Float.class.isAssignableFrom( clazz ) ) return Float.parseFloat( value );
    if( Double.class.isAssignableFrom( clazz ) ) return Double.parseDouble( value );
    return value;
}

and the above is not even the worse one I saw, so far :)

Does anybody have a secret trick here?