Java新手：<>和（）之间的区别？

Question

I'm learning Java and had a question about the difference between <> and (), such as when defining a class? For example:

public class CounterMap<K, V> implements java.io.Serializable {
   private static final long serialVersionUID = 11111;

   MapFactory<V, Double> mf;
   Map<K, Counter<V>> counterMap;

   protected Counter<V> ensureCounter(K key) {
       Counter<V> valueCounter = counterMap.get(key);
       if (valueCounter == null) {
           valueCounter = new Counter<V>(mf);
           counterMap.put(key, valueCounter);
       }
       return valueCounter;
   }
}

Any insight would be appreciated. Thanks.

Answer 1

Angle brackets < > are used to indicate generic types. For example, a list that contains Strings is type List<String> . Generics is an intermediate topic which - if you're a beginner - might be a little confusing, without first understanding other Java and programming basics.

Parentheses ( ) are used to invoke and declare methods, and they contain method parameters and arguments.

Your example is using generics to store any type of data in a map without having to be specific about what the type is. So if I wanted a CounterMap that stored key-value pairs of Long and String types, I could declare and initialize it like so:

CounterMap<Long, String> myCounterMap = new CounterMap<Long, String>();

Starting with Java 7, you can use something called the 'diamond' and simplify it to this:

CounterMap<Long, String> myCounterMap = new CounterMap<>();

Answer 2

Somewhat related.

parameter, variable, argument --

void f(Number n)  // define a parameter `n` of type `Number`
{
    // here, `n` is a variable. (JLS jargon: "parameter variable")
}

f(x);        // invoke with argument `x`, which must be a `Number`

type-parameter, type-variable, type-argument --

<N extends Number> f()   // type-parameter `N` with bound `Number`
{
    // here, `N` is a type-variable.
}

<Integer>f();      //  instantiate with type-argument `Integer`, which is a `Number`

my thoughts on these terms.