Java tips

Annotation

Annotation 提供了关于代码的元数据 (metadata), 但本身不会参与代码执行

Declare. @Retention -> When, @Target -> Where

@Retention(RetentionPolicy.RUNTIME)
@Target({Type})
@interface Foo {
  boolean value() default true;
}

Use

@Foo(false)
class Bar {

}

Get annotation

Bar.class.getAnnotation(Foo.class).value();

Type Signatures

Java log 会用一些简化的符号来表示方法的参数和返回值类型. 比如大写字母 I, J, F, D, V, ...等表示基础类型, L<class>; 表示类,

The JNI uses the Java VM’s representation of type signatures.

Java VM Type Signatures

Type Signature	Java Type
V	void
Z	boolean
B	byte
C	char
S	short
I	int
J	long
F	float
D	double
L fully-qualified-class;	fully-qualified-class
[ type	type[]
( arg-types ) ret-type	method type

For example, the Java method:

long f (int n, String s, int[] arr);

has the following type signature:

(ILjava/lang/String;[I)J

system properties & environment variables

system properties
- Java command line -Dname=value
- System.setProperty(String key, String value)
- System.getProperty(String key)
envrionment variables
- System.getenv(String name)

Code conventions

Debug

Idea > Run / Debug
mvnDebug, Idea > Remote JVM Debug

Decompile .class file

Visit procyon: https://github.com/mstrobel/procyon
Download procyon-decompiler-0.6.0.jar from Release tab

Run

java -jar procyon-decompiler-0.6.0.jar ~/Demo.class

Initialize a List

List<String> names = new ArrayList<>();

ArrayList 读取较快 a dynamically re-sizing array
LinkedList 修改较快 a doubly-linked list

UnsupportedOperation

Cause: Arrays.asList returns a fixed-size list

List<Integer> list1 = Arrays.asList(1, 2, 3);
list1.add(4); // Error

Solution:

List<Integer> list1 = new ArrayList<>(Arrays.asList(1, 2, 3));
list1.add(4); // Pass

多个文件编译运行

可以只编译运行入口文件(包含main的文件)。其它类，只要类名与文件名相同，会自动编译和引入这些类。

举例，有三个文件Deck.java, Suit.java, Rank.java。Deck.java 中使用了类Suit, Rank

javac Deck.java
java Deck

Run jar

java -jar example.jar

Stream Aggregate Operations

stream, filter, map, forEach, ...

persons
    .stream()
    .filter(p -> p.getGender() == Person.Sex.MALE)
    .map(p -> p.getEmailAddress())
    .forEach(email -> System.out.println(email));

Primitive data types and default values

Field variables

Data Type	Default Value (for fields)
byte	0
short	0
int	0
long	0L
float	0.0f
double	0.0d
char	'\u0000'
String (or any object)	null
boolean	false

Local variables

Local variables are uninitialized if you cannot initialize them.

Operator Precedence

Operators	Precedence
postfix	expr++ expr--
unary	++expr --expr +expr -expr ~ !
multiplicative	* / %
additive	+ -
shift	<< >> >>>
relational	< > <= >= instanceof
equality	== !=
bitwise AND	&
bitwise exclusive OR	^
bitwise inclusive OR	\|
logical AND	&&
logical OR	\|
ternary	? :
assignment	= += -= *= /= %= &= ^= \|= <<= >>= >>>=

Dependency Injection pattern

class Foo {
    private Database database;

    Foo() {
        this.database = new Database("/path/to/my/data");
    }
}

// Use dependency injection
class Foo {
    private Database database;

    Foo(Database database) {
        this.database = database
    }
}

可以使用guice，轻量级的 dependency injection framework.

Install java on ubuntu

JRE

<https://ubuntu.com/tutorials/install-jre#2-installing-openjdk-jre >

To install the OpenJDK JRE, we run

sudo apt update
sudo apt install default-jre

java --version

JDK

sudo apt install openjdk-11-demo openjdk-11-doc openjdk-11-jdk openjdk-11-source

Set `JAVA_HOME`

列出安装的 java 环境

sudo update-alternatives --config java

选择合适的，设置环境变量 JAVA_HOME

JAVA_HOME="/usr/lib/jvm/java-11-openjdk-amd64"

Glossary

JRE

Java software for your computer, or the Java Runtime Environment, is also referred to as the Java Runtime, Runtime Environment, Runtime, JRE, Java Virtual Machine, Virtual Machine, Java VM, JVM, VM, Java plug-in, Java plugin, Java add-on or Java download.

JDK

Java Development Kit, contains JRE.

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