Remember erasure ?

import java.util.ArrayList;
import java.util.List;

public class Main {

    public static void main(String[] args) {
        List<String> strings = new ArrayList<>();
        ((List) strings).add(42); // compiler warning - ok at runtime
        // String s = strings.get(0); //  CCE - Compiler-generated cast
        System.out.println("strings: " + strings); // strings: [42]
        System.out.println(strings.contains(42)); // true
    }
}

In other words:

    final HashSet<Integer> hashSet = new HashSet<>();
    // Set<String> cast = (Set<String>) hashSet; // error
    Set<String> cast = (Set) hashSet; // warning

From: here and here

Remember printing (and varargs) ?

Arrays.toString(map.entrySet().toArray());

as opposed to:

int[] a = { 1, 2, 3 }; System.out.println(Arrays.asList(a)); // \[\[I@70cdd2\] // List<int[]>

Remember generics ?

• How to implement an interface with an enum, where the interface extends Comparable?

  public interface Foo extends Comparable<Foo> {}
  public enum FooImpl implements Foo {}
  // java.lang.Comparable cannot be inherited with different arguments: <Foo> and <FooImpl>
  
  // Solution:
  public interface Foo<SelfType extends Foo<SelfType>> extends Comparable<SelfType> {}
  public enum FooImpl implements Foo<FooImpl> {}
  

• Why can’t you have multiple interfaces in a bounded wildcard generic?

Trivia

• Interface and Class Naming Anti-Patterns
• IAE vs NPE
• Why cast after an instanceOf?
• Should methods in a Java interface be declared with or without a public access modifier?
• .clone() or Arrays.copyOf()?