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javagenericstype-erasure

Get the number of generic type parameters of a class using its instance?

Is it possible to get the number of type parameters a class has/accepts in Java using its instance? For example,

Map<String, Integer> s = new HashMap<>();
int numParams = getNumOfTypeParameters(s); // Should return 2

I know that due to type erasure we can't get the type of these parameters but can we also not get the number of parameters?

Solution

  • It's as simple as

    System.out.println(Map.class.getTypeParameters().length); // 2

System.out.println(Class.class.getTypeParameters().length); // 1

System.out.println(Object.class.getTypeParameters().length); // 0

    Just look at the documentation for java.lang.Class and you'l find this.