10 things you did not know about Java

1. There are no checked exceptions.

public class Test {
    // Нету throws: исключения не объявлены
    public static void main(String[] args) {
        doThrow(new SQLException());
    }
    static void doThrow(Exception e) {
        Test. doThrow0(e);
    }
    @SuppressWarnings("unchecked")
    static  
    void doThrow0(Exception e) throws E {
        throw (E) e;
    }
}

2. You can create two methods that differ only in the return type

class Test {
    Object x() { return "abc"; }
    String x() { return "123"; }
}

Please note that a class may contain several suitable methods, because although the Java language prohibits declaring several methods with the same signature, the Java virtual machine still allows this if the return type is different. Such flexibility of a virtual machine can be used to implement some features of the language. For example, a covariant return type can be implemented using the bridge method, which differs from the real overloaded method only in the return type.

abstract class Parent {
    abstract T x();
}
class Child extends Parent {
    @Override
    String x() { return "abc"; }
}

// Method descriptor #15 ()Ljava/lang/String;
// Stack: 1, Locals: 1
java.lang.String x();
  0  ldc  [16]
  2  areturn
    Line numbers:
      [pc: 0, line: 7]
    Local variable table:
      [pc: 0, pc: 3] local: this index: 0 type: Child
// Method descriptor #18 ()Ljava/lang/Object;
// Stack: 1, Locals: 1
bridge synthetic java.lang.Object x();
  0  aload_0 [this]
  1  invokevirtual Child.x() : java.lang.String [19]
  4  areturn
    Line numbers:
      [pc: 0, line: 1]

3. These are all two-dimensional arrays!

class Test {
    int[][] a()  { return new int[0][]; }
    int[] b() [] { return new int[0][]; }
    int c() [][] { return new int[0][]; }
}

class Test {
    int[][] a = {{}};
    int[] b[] = {{}};
    int c[][] = {{}};
}

@Target(ElementType.TYPE_USE)
@interface Crazy {}
class Test {
    @Crazy int[][]  a1 = {{}};
    int @Crazy [][] a2 = {{}};
    int[] @Crazy [] a3 = {{}};
    @Crazy int[] b1[]  = {{}};
    int @Crazy [] b2[] = {{}};
    int[] b3 @Crazy [] = {{}};
    @Crazy int c1[][]  = {{}};
    int c2 @Crazy [][] = {{}};
    int c3[] @Crazy [] = {{}};
}

Type Annotations. Mysterious and powerful mechanism . Steeper than his mystery is perhaps his power .

My last commit before my monthly vacation

4. You do not understand conditional constructions

Object o1 = true ? new Integer(1) : new Double(2.0);

Object o2;
if (true)
    o2 = new Integer(1);
else
    o2 = new Double(2.0);

System.out.println(o1);
System.out.println(o2);

1.0
1

Integer i = new Integer(1);
if (i.equals(1))
    i = null;
Double d = new Double(2.0);
Object o = true ? i : d; // NullPointerException!
System.out.println(o);

5. You do not understand the compound assignment operator either

i += j;
i = i + j;

A compound assignment operator of the form E1 op = E2 is equivalent to the expression E1 = (T) ((E1) op (E2)), where T is a type of E1, except that E1 is calculated only once.

An example of such a type conversion can be shown at * = or / =

byte b = 10;
b * = 5.7;
System.out.println (b); // print 57

or

byte b = 100;
b / = 2.5;
System.out.println (b); // print 40

or

char ch = '0';
ch * = 1.1;
System.out.println (ch); // prints '4'

or

char ch = 'A';
ch * = 1.5;
System.out.println (ch); // print 'a'

6. Random integers

for (int i = 0; i < 10; i++) {
  System.out.println((Integer) i);
}

92
221
45
48
236
183
39
193
33
84

7. GOTO

int goto = 1;

Test.java:44: error:  expected
    int goto = 1;
        ^

label: {
  // ... какой-то код...
  if (check) break label;
  // ...ещё какой-то код...
}

2  iload_1 [check]
3  ifeq 6          // Прыжок вперёд
6  ..

label: do {
  // ... какой-то код...
  if (check) continue label;
  // ...ещё какой-то код...
  break label;
} while(true);

 2  iload_1 [check]
 3  ifeq 9
 6  goto 2          // Прыжок назад
 9  ..

8. Java has type aliases

interface People => Set;

People?      p1 = null;
Set? p2 = p1;
People?      p3 = p2;

class Test {
     void x(I i, L l) {
        System.out.println(
            i.intValue() + ", " + 
            l.longValue()
        );
    }
}

new Test().x(1, 2L);

// Вспомогательный тип. Можно использовать и просто List
interface Type {}
class C implements Type> {}
class D
 implements Type>>> {}

public abstract class Enum> { ... }

// Это
enum MyEnum {}
// На самом деле сахар для этого
class MyEnum extends Enum { ... }

class Test {
    Type c = new C();
    Type> d = new D();
}

class Test {
}

// Ошибка компиляции
Test s = null;
// Работает
Test d = null;

 void execute(T t) {}

execute((Serializable) (() -> {}));

execute((Runnable & Serializable) (() -> {}));