Hello world

Look at the code https://github.com/openjdk/jdk/blob/master/src/java.base/share/classes/java/lang/System.java to learn the coding style of java.

All primitive types, such as int, char, and double begin with a lowercase. All other types, such as String and Math, begin with an uppercase.

package java.lang;

// Class names looks like: MyClass
public final class System {}

// method name
private static native void registerNatives();

// constructor
private System() {}

// member variables
public static final InputStream in = null;
private static String notSupportedJnuEncoding;

// constants
private static final int NEVER = 1;

It using // and /**/ as comments, same as C++.

Hello.java

// Usage 1:
//   java Hello.java
// Usage 2:
//   javac Hello.java
//   java Hello
//
//  Note:
//   - "javac Hello.java" generates a file "Hello.class"
//   - "java Hello" takes as input "Hello.class" and executes it
//
// The class name Hello must match the filename Hello.java
// By convention, the class name looks like `MyClass`.
class Hello {
  // Note: How is array defined in java
  public static void main(String[] args) {
    System.out.println("hello world");
  }
} // There is no ';' here
// Every line of runnable code must be in some `class`

EqualTest.java

class EqualTest {
  public int i;

  public EqualTest(int a) {
    this.i = a;
  }

  public boolean equals(Object anObject) {
    if (this == anObject) {
      return true;
    }
    if (anObject instanceof EqualTest) {
      return this.i == ((EqualTest) anObject).i;
    }
    return false;
  }

  public static void main(String[] args) {
    EqualTest e1 = new EqualTest(10);
    EqualTest e2 = new EqualTest(10);

    System.out.println(e1 == e2); // false, compare the reference
    System.out.println(e1 != e2); // true
    System.out.println(e1.equals(e2)); // true, compare the contained value
  }
}

Variables.java

class Variables {
  public static void main(String[] args) {
    String msg = "Hello world";
    int i = 10;
    i++; // ok
    ++i; // ok
    // i-- // error, should be --i
    --i;
    int ii = i % 2;
    i = 10;
    ii = i / 3; // truncated
    System.out.println(ii); // 3
    ii += 2;
    System.out.println(ii); // 5
    // also has:
    // -=, &=, |=, >>=, <<=, ^=, /=, *=
    // Similar to C/C++

    /* float f = 1.25; // error: possible lossy conversion from double to float */
    // Caution: We have to use `1.25f` instead of `1.25` to assign a float variable
    float f = 1.25f;
    double d = 1.25;
    d = 1.25d; // also ok

    // f = d; // error: possible lossy conversion from double to float
    f = (float) d; // ok, explicit cast
    d = f; // ok

    char c = 'h'; // a character

    i = 10;
    System.out.println(msg);
    System.out.println(i); // 10
    System.out.println(f); // 1.25
    System.out.println(d); // 1.25
    System.out.println(c); // h

    // define multiple variable in the same line
    int i1 = 1, i2;
    // System.out.println(i2); // variable i2 might not have been initialized
    i2 = 3;
    System.out.println(i2); // 3
    boolean b = false;
    System.out.println(b); // false
    b = true;
    // b = 1; // error: int cannot be converted to boolean
    System.out.println(b); // true

    // byte b0 = 128; // error: lossy conversion from int to byte
    byte b0 = 127; // ok
    /* b0 = -129; // error: lossy conversion from int to type */
    b0 = -128; // ok

    // short s0 = 32768; // error: lssoy conversion from int to short
    short s0 = 32767; // ok
    // s0 = -32769; // error: lossy conversion fro int to short
    s0 = -32768; // ok

    // other types: int, long, float, double, boolean, char
    // Note: char has 2-byte
    // boolean has 1-bit
    // There are no unsigned integers !!

    // ternary operator :?
    System.out.println(2 > 3 ? "yes" : "no"); // no
  }
}

Strings.java

class Strings {
  public static void main(String[] args) {
    String s = "abc";
    System.out.println(s.length()); // 3
    System.out.println(s.toUpperCase()); // ABC
    System.out.println(s.toUpperCase().toLowerCase()); // abc
    System.out.println(s.indexOf("ab")); // 0
    System.out.println(s.indexOf("a")); // 0
    System.out.println(s.indexOf("bc")); // 1
    System.out.println(s.indexOf("bca")); // when not found, it is -1

    System.out.println(s.charAt(0) == 'a'); // true
    System.out.println(s + "123" == "abc123"); // false, == compares the reference
    System.out.println((s + "123").equals("abc123")); // true, compare the content
    System.out.println(s.concat("123").equals("abc123")); // true, compare the content
    System.out.println((s + 123).equals("abc123")); // true, compare the content
    System.out.println((123 + s).equals("123abc")); // true, compare the content
    s = "a\"b";
  }
}
;

MathTest.java

class MathTest {
  public static void main(String[] args) {
    System.out.println(Math.max(2, 3)); // 3
    System.out.println(Math.max(2, 3.5)); // 3.5
    System.out.println(Math.min(2, 3.5)); // 2
    System.out.println(Math.sqrt(6.25)); // 2.5
    System.out.println(Math.sqrt(9)); // 3.0, note it is a double
    System.out.println(Math.abs(-9)); // -9
    System.out.println(Math.abs(-9.0)); // -9.0
    System.out.println(Math.random()); // a number in the range [0, 1)
  }
}

ArrayTest.java

class ArrayTest {
  public static void main(String[] args) {
    int[] a = {1, 2, 3};
    for (int i : a) {
      System.out.println(i);
    }
    /*
    1
    2
    3
     */
    System.out.println(a[0]); // 1
    System.out.println(a.length); // 3
    for (int i = 0; i != a.length; ++i) {
      System.out.println(a[i]);
    }
    /*
    1
    2
    3
         */
  }
}

TODOs

How to define an array? How to iterate an array?
What methods does the String class have
What are final variables?
How to define a package?
What is static import?
What is a lambda expression?
How to write comments using javadoc?
How to generate documentation from javadoc?
How to use a for-each loop?
How to import a class from another file?