How do I clear system property?

By Wayan in Core API, Lang Package 0 Comment

The System.clearProperty(String key) method enables you to remove a system property. The key must not be an empty string or a null value because it will cause the method to throw an IllegalArgumentException or a NullPointerException.

It will also check if a SecurityManager exists and if you don’t have a write permission to the system property a SecurityException is going to be thrown.

package org.kodejava.lang;

public class ClearProperty {
    public static void main(String[] args) {
        String key = "user.dir";
        System.out.println(key + " = " + System.getProperty(key));

        // The System.clearProperty() method available since Java 1.5
        System.clearProperty(key);
        System.out.println(key + " = " + System.getProperty(key));
    }
}

The code snippet above give us the following output:

user.dir = F:\Wayan\Kodejava\kodejava-example
user.dir = null

How do I get file separator symbol?

By Wayan in Core API, Lang Package 0 Comment

Creating a program to be run on more than one platform such as Windows and Linux our program need to understand the difference between both platform. The simplest thing for instance is the file separator. Windows use "\" (backslash) while Linux use "/" (forward slash).

To avoid manual checking for the operating system we can get the file separator symbol from the system property using the file.separator key.

package org.kodejava.lang;

public class FileSeparatorExample {
    public static void main(String[] args) {
        // file.separator system property return the correct file 
        // separator for each different platform (Windows = \), 
        // (Linux = /)
        String dataFolder = System.getProperty("user.dir") +
                System.getProperty("file.separator") + "data";

        System.out.println("Data Folder = " + dataFolder);
    }
}

How do I get the name of a class?

By Wayan in Core API, Lang Package 0 Comment 
package org.kodejava.lang;

import java.util.Calendar;
import java.math.BigDecimal;

public class ClassName {
    public static void main(String[] args) {
        // Get the name of the classes below.
        Class<?> clazz = String.class;
        System.out.println("Class Name: " + clazz.getName());

        clazz = Calendar.class;
        System.out.println("Class Name: " + clazz.getName());

        clazz = BigDecimal.class;
        System.out.println("Class Name: " + clazz.getName());
    }
}

The result of the code snippet above are:

Class Name: java.lang.String
Class Name: java.util.Calendar
Class Name: java.math.BigDecimal

How do I know if a class object is an interface or a class?

By Wayan in Core API, Lang Package 0 Comment 
package org.kodejava.lang;

public class CheckForInterface {
    public static void main(String[] args) {
        // Checking whether Cloneable is an interface or class
        Class<?> clazz = Cloneable.class;
        boolean isInterface = clazz.isInterface();
        System.out.println(clazz.getName() + " is an interface = " + isInterface);

        // Checking whether String is an interface or class
        clazz = String.class;
        isInterface = clazz.isInterface();
        System.out.println(clazz.getName() + " is an interface = " + isInterface);
    }
}

java.lang.Cloneable is an interface = true
java.lang.String is an interface = false

How do I get interfaces implemented by a class?

By Wayan in Core API, Lang Package 0 Comment

The getClass().getInterfaces() method return an array of Class that represents the interfaces implemented by an object.

package org.kodejava.lang;

import java.util.Date;
import java.util.Calendar;

public class ClassInterfaces {
    public static void main(String[] args) {
        // Get an instance of Date class
        Date date = Calendar.getInstance().getTime();

        // Get all interfaces implemented by the java.util.Date class and
        // print their names.
        Class<?>[] interfaces = date.getClass().getInterfaces();
        for (Class<?> i : interfaces) {
            System.out.printf("Interface of %s = %s%n", date.getClass().getName(), i.getName());
        }

        // For the primitive type the interface will be an empty array.
        interfaces = char.class.getInterfaces();
        for (Class<?> i : interfaces) {
            System.out.printf("Interface of %s = %s%n", char.class.getName(), i.getName());
        }
    }
}

The java.util.Date class implements the following interfaces:

Interface of java.util.Date = java.io.Serializable
Interface of java.util.Date = java.lang.Cloneable
Interface of java.util.Date = java.lang.Comparable

How do I get package name of a class?

By Wayan in Core API, Lang Package 0 Comment

To get the package name of a class we can use the getClass().getPackage() method of the inspected object.

package org.kodejava.lang;

import java.util.Date;

public class GetPackageName {
    public static void main(String[] args) {
        // Create an instance of Date class, and then obtain the class package
        // name.
        Date date = new Date();
        Package pack = date.getClass().getPackage();
        String packageName = pack.getName();
        System.out.println("Package Name = " + packageName);

        // Create an instance of our class and again get its package name
        GetPackageName object = new GetPackageName();
        packageName = object.getClass().getPackage().getName();
        System.out.println("Package Name = " + packageName);
    }
}

Our code printed the following output:

Package Name = java.util
Package Name = org.kodejava.lang

How do I get super class of an object?

By Wayan in Core API, Lang Package 0 Comment

To get the super class of an object we can call the object’s getClass() method. After getting the class type of the object we call the getSuperclass() method to get the superclass. Let’s see the code snippet below.

package org.kodejava.lang;

public class ObtainingSuperClass {
    public static void main(String[] args) {
        // Create an instance of String class
        Object object1 = new String("Hello");

        // Get String class super class
        Class<?> clazz1 = object1.getClass().getSuperclass();
        System.out.println("Super Class = " + clazz1);

        // Create an instance of StringIndexOutOfBoundsException class
        Object object2 = new StringIndexOutOfBoundsException("Error message");

        // Get StringIndexOutOfBoundsException class super class
        Class<?> clazz2 = object2.getClass().getSuperclass();
        System.out.println("Super Class = " + clazz2);
    }
}

The program above prints the following string:

Super Class = class java.lang.Object
Super Class = class java.lang.IndexOutOfBoundsException

How do I read system property as an integer?

By Wayan in Core API, Lang Package 0 Comment

The Integer.getInteger() methods allows us to easily read system property and convert it directly to Integer object. This method call the System.getProperty() method and then convert it to integer by calling the Integer.decode() method.

package org.kodejava.lang;

public class IntegerProperty {
    public static void main(String[] args) {
        // Add properties to the system. In this example we create a
        // dummy major and minor version for our application.
        System.setProperty("app.major.version", "2021");
        System.setProperty("app.minor.version", "9");

        // In the code below we use the Integer.getInteger() method to
        // read our application version from the value specified in the
        // system properties.
        Integer major = Integer.getInteger("app.major.version");
        Integer minor = Integer.getInteger("app.minor.version");
        System.out.println("App version = " + major + "." + minor);
    }
}

The output of the code snippet:

App version = 2021.9

How do I decode string to integer?

By Wayan in Core API, Lang Package 0 Comment

The static Integer.decode() method can be used to convert a string representation of a number into an Integer object. Under the cover this method call the Integer.valueOf(String s, int radix).

The string can start with the optional negative sign followed with radix specified such as 0x, 0X, # for hexadecimal value, 0 (zero) for octal number.

package org.kodejava.lang;

public class IntegerDecode {
    public static void main(String[] args) {
        String decimal = "10"; // Decimal
        String hex = "0XFF"; // Hex
        String octal = "077"; // Octal

        Integer number = Integer.decode(decimal);
        System.out.println("String [" + decimal + "] = " + number);

        number = Integer.decode(hex);
        System.out.println("String [" + hex + "] = " + number);

        number = Integer.decode(octal);
        System.out.println("String [" + octal + "] = " + number);
    }
}

The result of the code snippet above:

String [10] = 10
String [0XFF] = 255
String [077] = 63

How do I insert a string in the StringBuilder?

By Wayan in Core API, Lang Package 0 Comment 
package org.kodejava.lang;

public class StringBuilderInsert {
    public static void main(String[] args) {
        StringBuilder alphabets = new StringBuilder("abcdfghopqrstuvwxyz");
        System.out.println("alphabets = " + alphabets);

        //  |a|b|c|d|f|g|h|i|....
        //  0|1|2|3|4|5|6|7|8|...
        //
        // From the above sequence you can see that the index of the string is
        // started from 0, so when we insert a string in the fourth offset it
        // means it will be inserted after the "d" letter. There are other overload
        // version of this method that can be used to insert other type of data
        // such as char, int, long, float, double, Object, etc.
        alphabets.insert(4, "e");
        System.out.println("alphabets = " + alphabets);

        // Here we insert an array of characters to the StringBuilder.
        alphabets.insert(8, new char[] {'i', 'j', 'k', 'l', 'm', 'n'});
        System.out.println("alphabets = " + alphabets);
    }
}

The result of the code snippet above:

alphabets = abcdfghopqrstuvwxyz
alphabets = abcdefghopqrstuvwxyz
alphabets = abcdefghijklmnopqrstuvwxyz