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Java notes for non-Java programmers

Java (1995) has C-like syntax with curly braces. It's an imperative, garbage collecting language with developed C++-like OOP. New versions support lambdas. Very similar to C#.

As a whole Java syntax is very stable, and many features are implemented as classes instead of syntax, for example, Optional.

Java has extensive standard library.

Related languages: Kotlin, C#, JavaScript, C++.

Fragmented type system

There're two category of types:

  • primitive types
  • reference types

Primitive types have value semantics.

Primitive types are:

boolean, byte, short, int, long, float, double, char

A programmer can't create new primitive types.

All numeric types in Java are signed! (char is unsigned.)

char is a single 16-bit Unicode character, as in C# and JavaScript. So Java has good support of Unicode out of the box.

TOADD: records

All other types are reference types. For example, Object, String, ArrayList.

There's also distinct type category for C-like arrays of fixed-size. Arrays have reference semantics also.

Literals

Literals for primitive types:

true false
123_456 123_456L (long) (there's no `byte` or `short` suffix)
0xfe, 0b1010
+123.0e10 (double), 123.0f, 123.0d (exists, but not needed)
'a', 'b', 'c'

Literals for reference types:

"hello"
null

Special characters:

\b (backspace)
\t (tab)
\n (line feed)
\f (form feed)
\r (carriage return)
\" (double quote)
\' (single quote)
\\ (backslash)

There're also weird special class literals:

Object.class
Integer.class
String.class
ArrayList.class

Fixed-size arrays

int[] a = { 11, 22, 33 };
a.length
a[0], a[1], a[2]

Note that array length is a special attribute, so no ().

No slice syntax. No negative indices.

TOCHECK: No methods.

Strings

Strings are immutable.

Strings has no indexed access, but there is charAt method.

Strings can be null.

s.length()

Operator == compares strings' identities, not values (but there's equals method).

s.equals(t)

Definite initialization analysis

Everything but local variables is zero-initialized (default-initialized) by default.

`false`,  `0`, `+0.0`,`'\0'`, `null`

Java does definite initialization (DI) analysis.

final variables can obtain value after introduction, or in different code paths thanks to DI analysis.

Java requires all code path to have return-like statement in a function.

For uninitialized variables Java has definite initialization analys, used also for final local variables.

Control flow

switch

There're two switch construct syntaxes: C-like and shortened switch statement/expresion:

int x = 1;
String s = switch (x) {
    case 1 -> "one";
    case 2 -> "two";
    default -> "?";
};

Generics and type erasure

Generics in Java are checked on compile time and then normalized to use Object internally. It's called type erasure. So there is no reification of templated code.

TOCHEK: Generics on primitive types are impossible, due to type erasure, because primitive types are not convertable to Object easily. But one can write generic code on boxed primitives, like Boolean, Character, Integer.

static methods aren't generic by default in a generic class

Static methods do not inherit genericness of a class. To make a static method generic, one can use the following syntax:

public static <T> Optional<T> of(T value) {
}

Function references

ArrayList::size

TODO