So, you’ve been working with Java since the very beginning? Remember the days when it was called “Oak”, when OO was still a hot topic, when C++ folks thought that Java had no chance, when Applets were still a thing?
I bet that you didn’t know at least half of the following things. Let’s start this week with some great surprises about the inner workings of Java.
1. There is no such thing as a checked exception
That’s right! The JVM doesn’t know any such thing, only the Java language does.
Today, everyone agrees that checked exceptions were a mistake. As Bruce Eckel said on his closing keynote at GeeCON, Prague, no other language after Java has engaged in using checked exceptions, and even Java 8 does no longer embrace them in the new Streams API (which can actually be a bit of a pain, when your lambdas use IO or JDBC).
Do you want proof that the JVM doesn’t know such a thing? Try the following code:
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| public class Test { // No throws clause here public static void main(String[] args) { doThrow(new SQLException()); } static void doThrow(Exception e) { Test. } @SuppressWarnings("unchecked") static void doThrow0(Exception e) throws E { throw (E) e; }} |
Not only does this compile, this also actually throws the
SQLException, you don’t even need Lombok’s @SneakyThrows for that.2. You can have method overloads differing only in return types
That doesn’t compile, right?
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| class Test { Object x() { return "abc"; } String x() { return "123"; }} |
Right. The Java language doesn’t allow for two methods to be “override-equivalent” within the same class, regardless of their potentially differing
throws clauses or return types.
But wait a second. Check out the Javadoc of
Class.getMethod(String, Class...). It reads:Note that there may be more than one matching method in a class because while the Java language forbids a class to declare multiple methods with the same signature but different return types, the Java virtual machine does not. This increased flexibility in the virtual machine can be used to implement various language features. For example, covariant returns can be implemented with bridge methods; the bridge method and the method being overridden would have the same signature but different return types.
Wow, yes that makes sense. In fact, that’s pretty much what happens when you write the following:
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| abstract class Parent abstract T x();}class Child extends Parent @Override String x() { return "abc"; }} |
Check out the generated byte code in
Child:
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| // Method descriptor #15 ()Ljava/lang/String;// Stack: 1, Locals: 1java.lang.String x(); 0 ldc 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: 1bridge 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] |
So,
T is really just Object in byte code. That’s well understood.
The synthetic bridge method is actually generated by the compiler because the return type of the
Parent.x() signature may be expected to Object at certain call sites. Adding generics without such bridge methods would not have been possible in a binary compatible way. So, changing the JVM to allow for this feature was the lesser pain (which also allows covariant overriding as a side-effect…) Clever, huh?
Are you into language specifics and internals? Then find some more very interesting details here.
3. All of these are two-dimensional arrays!
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| class Test { int[][] a() { return new int[0][]; } int[] b() [] { return new int[0][]; } int c() [][] { return new int[0][]; }} |
Yes, it’s true. Even if your mental parser might not immediately understand the return type of the above methods, they are all the same! Similar to the following piece of code:
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| class Test { int[][] a = {{}}; int[] b[] = {{}}; int c[][] = {{}};} |
You think that’s crazy? Imagine using JSR-308 / Java 8 type annotations on the above. The number of syntactic possibilities explodes!
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| @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 [] = {{}};} |
4. You don’t get the conditional expression
So, you thought you knew it all when it comes to using the conditional expression? Let me tell you, you didn’t. Most of you will think that the below two snippets are equivalent:
Object o1 =true?newInteger(1) :newDouble(2.0);… the same as this?
Object o2;if(true)o2 =newInteger(1);elseo2 =newDouble(2.0);Nope. Let’s run a quick test
System.out.println(o1);System.out.println(o2);This programme will print:1.0 1Yep! The conditional operator will implement numeric type promotion, if “needed”, with a very very very strong set of quotation marks on that “needed”. Because, would you expect this programme to throw aNullPointerException?
Integer i =newInteger(1);if(i.equals(1))i =null;Double d =newDouble(2.0);Object o =true? i : d;// NullPointerException!System.out.println(o);5. You also don’t get the compound assignment operator
Quirky enough? Let’s consider the following two pieces of code:
i += j;i = i + j;Intuitively, they should be equivalent, right? But guess what. They aren’t! The JLS specifies:A compound assignment expression of the form E1 op= E2 is equivalent to E1 = (T)((E1) op (E2)), where T is the type of E1, except that E1 is evaluated only once.This is so beautiful, I would like to cite Peter Lawrey‘s answer to this Stack Overflow question:A good example of this casting is using *= or /=byte b = 10; b *= 5.7; System.out.println(b); // prints 57orbyte b = 100; b /= 2.5; System.out.println(b); // prints 40orchar ch = '0'; ch *= 1.1; System.out.println(ch); // prints '4'orchar ch = 'A'; ch *= 1.5; System.out.println(ch); // prints 'a'Now, how incredibly useful is that? I’m going to cast/multiply chars right there in my application. Because, you know…6. Random integers
Now, this is more of a puzzler. Don’t read the solution yet. See if you can find this one out yourself. When I run the following programme:
for(inti =0; i <10; i++) {System.out.println((Integer) i);}… then “sometimes”, I get the following output:92 221 45 48 236 183 39 193 33 84How is that even possible??...... spoiler… solution ahead….....OK, it has to do with overriding the JDK’sIntegercache via reflection, and then using auto-boxing and auto-unboxing. Don’t do this at home! Or in other words, let’s think about it this way, once more
7. GOTO
This is one of my favourite. Java has GOTO! Type it…
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| int goto = 1; |
This will result in:
Test.java:44: error:expected int goto = 1; ^
This is because
goto is an unused keyword, just in case…
But that’s not the exciting part. The exciting part is that you can actually implement goto with
break, continue and labelled blocks:
Jumping forward
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| label: { // do stuff if (check) break label; // do more stuff} |
In bytecode:
2 iload_1 [check] 3 ifeq 6 // Jumping forward 6 ..
Jumping backward
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| label: do { // do stuff if (check) continue label; // do more stuff break label;} while(true); |
In bytecode:
2 iload_1 [check] 3 ifeq 9 6 goto 2 // Jumping backward
8. Java has type aliases
In other languages (e.g. Ceylon), we can define type aliases very easily:
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| interface People => Set |
A
People type constructed in such a way can then be used interchangably with Set:
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| People? p1 = null;SetPeople? p3 = p2; |
In Java, we can’t define type aliases at a top level. But we can do so for the scope of a class, or a method. Let’s consider that we’re unhappy with the namings of
Integer, Longetc, we want shorter names: I and L. Easy:
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| class Test System.out.println( i.intValue() + ", " + l.longValue() ); }} |
In the above programme,
Integer is “aliased” to I for the scope of the Test class, whereas Long is “aliased” to L for the scope of the x() method. We can then call the above method like this:
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| new Test().x(1, 2L); |
This technique is of course not to be taken seriously. In this case,
Integer and Long are both final types, which means that the types I and L are effectively aliases (almost. assignment-compatibility only goes one way). If we had used non-final types (e.g. Object), then we’d be really using ordinary generics.
Enough of these silly tricks. Now for something truly remarkable!
9. Some type relationships are undecidable!
OK, this will now get really funky, so take a cup of coffee and concentrate. Consider the following two types:
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| // A helper type. You could also just use Listinterface Typeclass C implements Typeclass D implements Type |
Now, what do the types
C and D even mean?
They are somewhat recursive, in a similar (yet subtly different) way that
java.lang.Enumis recursive. Consider:
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| public abstract class Enum |
With the above specification, an actual
enum implementation is just mere syntactic sugar:
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| // Thisenum MyEnum {}// Is really just sugar for thisclass MyEnum extends Enum |
With this in mind, let’s get back to our two types. Does the following compile?
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| class Test { Type Type} |
Hard question, and Ross Tate has an answer to it. The question is in fact undecidable:
Is C a subtype of Type?
Step 0) C Step 1) Type> And then:Is D a subtype of Type>?Step 0) D> Step 1) Type >>> > Step 2) D >> Step 3) Type >> > Step 4) D > >> Step . . . (expand forever) Try compiling the above in your Eclipse, it’ll crash!
10. Type intersections
classTestextendsSerializable & Cloneable> {}The generic type parameterTthat you’re binding to instances of the classTestmust implement bothSerializableandCloneable. For instance,Stringis not a possible bound, butDateis:
// Doesn't compileTests = null;// CompilesTestd = null;This feature has seen reuse in Java 8, where you can now cast types to ad-hoc type intersections. How is this useful? Almost not at all, but if you want to coerce a lambda expression into such a type, there’s no other way. Let’s assume you have this crazy type constraint on your method:
extendsRunnable & Serializable>voidexecute(T t) {}You want aRunnablethat is alsoSerializablejust in case you’d like to execute it somewhere else and send it over the wire. Lambdas and serialisation are a bit of a quirk.You can serialize a lambda expression if its target type and its captured arguments are serializableBut even if that’s true, they do not automatically implement theSerializablemarker interface. To coerce them to that type, you must cast. But when you cast only toSerializable…
execute((Serializable) (() -> {}));… then the lambda will no longer be Runnable.Egh…So…Cast it to both types:
execute((Runnable & Serializable) (() -> {}));
Conclusion
