[Basic knowledge of Java] About type conversion

Introduction

This article is --Start learning Java ――I want to review the basics

For those who say.

The types and variables are described in the following articles. [Basic knowledge of Java] About types and variables

Basic type (primitive type) conversion

Here's how to convert a variable of one basic type to a variable of another basic type.

Small to large mold

The integer data type is byte < short < int < long It is a feeling of size. When converting from a small type to a large type, there are ** when the type is automatically converted ** and ** a cast where the programmer intentionally converts.

When the type is converted automatically

    byte a = 127;
    short b = a;
    System.out.println(b);//Output result = 127

    short c = 32767;
    long d = c;
    System.out.println(d);//Output result = 32767

A cast where the programmer consciously transforms

    byte a = 127;
    byte b = 127;
    System.out.println((long)a + b);//Output result = 254

    int c = 2147483647;
    int d = 2147483647;
    System.out.println((long)c + d);//Output result = 4294967294

Large to small mold

Be careful when converting from large to small types.

I get an error if I do the following.

    long a = Long.MAX_VALUE;//Substituting the maximum value of long
    System.out.println(a);//Output result = 9223372036854775807
    short b = a;//Type mismatch: cannot convert from long to short

There is a way to fix the error, but the contents of the variable will change. If the size exceeds the size that the data type can have, data will be truncated (data overflow), and the expected execution result will not be obtained.

    long a = Long.MAX_VALUE;//Substituting the maximum value of long
    System.out.println(a);//Output result = 9223372036854775807
    short b = (short) a;//Type conversion by cast
    System.out.println(b);//Output result =-1

Reference type conversion

This time, I will only explain the type conversion of class types. When the reference type of the conversion source is a class type, the reference type of the conversion destination and the rules of type conversion are as shown in the figure below.

** Reference type implicit type conversion ** Parent class type variable = new child class ();

//Parent class
class Oya {
  void greet() {
    String a = "Good morning";
      System.out.println(a);
  }
}

//Child class that inherits the parent class
class Kodomo extends Oya {
  void name() {
    String b = "Iniesta";
      System.out.println(b);
  }
  public static void main(String[] args) {
      Oya oy = new Kodomo();
      oy.greet();//Output result = good morning
      Oya oy2 = new Oya();
      oy2.greet();//Output result = good morning

      Kodomo ko = new Kodomo();
      ko.greet();//Output result = good morning
      ko.name();//Output result = Iniesta
      Kodomo ko2 = new Oya();//Error: Type mismatch: cannot convert from Oya to Kodomo

  }
}

As you can see in the error Child class type variable = new parent class (); You can not.

Conversion to string

It is possible to convert from all types to character types.

    Integer a1 = 123;
    String s1 = "Ah" + a1;
    System.out.println(s1);//Output result = A 123
    System.out.println(s1 instanceof String);//Output result = true


    int a2 = -123;
    String s2 = String.valueOf(a2);
    System.out.println(s2);//Output result =-123
    System.out.println(s2 instanceof String);//Output result = true

Finally

I still don't have a deep understanding of reference type conversion, so I will add it as soon as I understand it.

Then thank you ☕️