How do I get environment variables?

By Wayan in Core API, Lang Package 0 Comment

Environment variables are a set of dynamic values that can affect a running process, such as our Java program. Each process usually have their own copy of these variables.

Now we would like to obtain the available variables in our environment or operating system, how do I do this in Java? Here is a code example of it.

package org.kodejava.lang;

import java.util.Map;
import java.util.Set;

public class SystemEnv {
    public static void main(String[] args) {
        // We get the environment information from the System class. 
        // The getenv method (why shouldn't it called getEnv()?) 
        // returns a map that will never have null keys or values 
        // returned.
        Map<String, String> map = System.getenv();

        Set<String> keys = map.keySet();
        for (String key : keys) {
            // Here we iterate based on the keys inside the map, and
            // with the key in hand we can get it values.
            String value = map.get(key);
            System.out.println(key + " = " + value);
        }
    }
}

Here are some results on my machine.

...
M2 = C:\ProgramData\chocolatey\lib\maven\apache-maven-3.5.0\bin
JETTY_HOME = C:\ProgramData\chocolatey\lib\jetty\tools\jetty-distribution-9.4.31.v20200723
USERNAME = wsaryada
ProgramFiles(x86) = C:\Program Files (x86)
M2_REPO = C:\Users\wsaryada\.m2
SPRING_HOME = C:\ProgramData\chocolatey\lib\spring-boot-cli\spring-2.2.4.RELEASE
M2_HOME = C:\ProgramData\chocolatey\lib\maven\apache-maven-3.5.0
JAVA_HOME = C:\Program Files\Java\jdk1.8.0_161
OS = Windows_NT
COMPUTERNAME = KRAKATAU
CATALINA_HOME = C:\tools\apache-tomcat-10.0.11
...

How do I copy some items of an array into another array?

By Wayan in Core API, Lang Package 0 Comment

In the code snippet below we are going to learn how to use the System.arraycopy() method to copy array values. This method will copy array from the specified source array, starting from the element specified by starting position. The array values will be copied to the target array, placed at the specified start position in the target array and will copy values for the specified number of elements.

The code snippet below will copy 3 elements from the letters array and place it in the results array. The copy start from the third elements which is the letter U and will be place at the beginning of the target array.

package org.kodejava.lang;

public class ArrayCopy {
    public static void main(String[] args) {
        // Creates letters array with 5 chars inside it.
        String[] letters = {"A", "I", "U", "E", "O"};

        // Create an array to where we are going to copy
        // some elements of the previous array.
        String[] results = new String[3];

        // Copy 3 characters from letters starting from the
        // third element and put it inside result array
        // beginning at the first element
        System.arraycopy(letters, 2, results, 0, 3);

        // Just print out what were got copied, it will
        // contains U, E, and O
        for (String result : results) {
            System.out.println("result = " + result);
        }
    }
}

The output of the code snippet:

result = U
result = E
result = O

How do I terminate a Java application?

By Wayan in Core API, Lang Package 0 Comment

In an application we sometimes want to terminate the execution of our application, for instance because it cannot find the required resource.

To terminate it we can use exit(status) method in java.lang.System class or in the java.lang.Runtime class. When terminating an application we need to provide a status code, a non-zero status assigned for any abnormal termination.

package org.kodejava.lang;

import java.io.File;

public class AppTerminate {
    public static void main(String[] args) {
        File file = new File("config.xml");

        int errCode = 0;
        if (!file.exists()) {
            errCode = 1;
        }

        // When the error code is not zero go terminate
        if (errCode > 0) {
            System.exit(errCode);
        }
    }
}

The call to System.exit(status) is equals to Runtime.getRuntime().exit(status). Actually the System class will delegate the termination process to the Runtime class.

Running the code give the following output because it can’t find the config.xml file.

Process finished with exit code 1

How do I check if a string is a valid number?

By Wayan in Core API, Lang Package 0 Comment

When building a computer program we will use a lot of string to represent our data. The data might not just information about our customer name, email or address, but will also contain numeric data represented as string. So how do we know if this string contains a valid number?

Java provides some wrappers to the primitive data types that can be used to do the checking. These wrappers come with the parseXXX() method such as Integer.parseInt(), Float.parseFloat() and Double.parseDouble() methods.

package org.kodejava.lang;

public class NumericParsingExample {
    public static void main(String[] args) {
        String age = "15";
        String height = "160.5";
        String weight = "55.9";

        try {
            int theAge = Integer.parseInt(age);
            float theHeight = Float.parseFloat(height);
            double theWeight = Double.parseDouble(weight);

            System.out.println("Age    = " + theAge);
            System.out.println("Height = " + theHeight);
            System.out.println("Weight = " + theWeight);
        } catch (NumberFormatException e) {
            e.printStackTrace();
        }
    }
}

In the example code we use Integer.parseInt(), Float.parseFloat(), Double.parseDouble() methods to check the validity of our numeric data. If the string is not a valid number java.lang.NumberFormatException will be thrown.

The result of our example:

Age    = 15
Height = 160.5
Weight = 55.9

How do I get the minimum and maximum value of a primitive data types?

By Wayan in Core API, Lang Package 0 Comment

To get the minimum or maximum value of a primitive data types such as byte, short, int, long, float and double we can use the wrapper class provided for each of them, the wrapper classes are Byte, Short, Integer, Long, Float and Double which is located in java.lang package.

package org.kodejava.lang;

public class MinMaxExample {
    public static void main(String[] args) {
        System.out.println("Byte.MIN    = " + Byte.MIN_VALUE);
        System.out.println("Byte.MAX    = " + Byte.MAX_VALUE);
        System.out.println("Short.MIN   = " + Short.MIN_VALUE);
        System.out.println("Short.MAX   = " + Short.MAX_VALUE);
        System.out.println("Integer.MIN = " + Integer.MIN_VALUE);
        System.out.println("Integer.MAX = " + Integer.MAX_VALUE);
        System.out.println("Long.MIN    = " + Long.MIN_VALUE);
        System.out.println("Long.MAX    = " + Long.MAX_VALUE);
        System.out.println("Float.MIN   = " + Float.MIN_VALUE);
        System.out.println("Float.MAX   = " + Float.MAX_VALUE);
        System.out.println("Double.MIN  = " + Double.MIN_VALUE);
        System.out.println("Double.MAX  = " + Double.MAX_VALUE);
    }
}

The result of the code above shows the minimum and maximum value for each data types.

Byte.MIN    = -128
Byte.MAX    = 127
Short.MIN   = -32768
Short.MAX   = 32767
Integer.MIN = -2147483648
Integer.MAX = 2147483647
Long.MIN    = -9223372036854775808
Long.MAX    = 9223372036854775807
Float.MIN   = 1.4E-45
Float.MAX   = 3.4028235E38
Double.MIN  = 4.9E-324
Double.MAX  = 1.7976931348623157E308

How do I convert primitive data types into String?

By Wayan in Core API, Lang Package 0 Comment

There are times when we want to convert data from primitive data types into a string, for instance when we want to format output on the screen or simply mixing it with other string. Using a various static method String.valueOf() we can get a string value of them.

Here is the code sample:

package org.kodejava.lang;

public class StringValueOfExample {
    public static void main(String[] args) {
        boolean b = false;
        char c = 'c';
        int i = 100;
        long l = 100000;
        float f = 3.4f;
        double d = 500.99;

        String u = String.valueOf(b);
        String v = String.valueOf(c);
        String w = String.valueOf(i);
        String x = String.valueOf(l);
        String y = String.valueOf(f);
        String z = String.valueOf(d);
    }
}

When called with boolean argument the String.valueOf() method return true or false string depending on the boolean argument value. When called with char argument, a 1 length sized string returned.

For int, long, float, double the results are the same as calling Integer.toString(), Long.toString(), Float.toString() and Double.toString() respectively.

How do I check a string starts with a specific word?

By Wayan in Core API, Lang Package 0 Comment

To test if a string starts with a specific prefix we can use the String.startsWith(String prefix) method. This method returns a boolean true as the result if the string starts with that specified prefix. The String.startsWith() method checks the prefix in case-sensitive manner.

The following code snippet give us a small example how to use this method, but instead of just straightly checks using the startsWith() method, we add a scanner that allows us to input the prefix that we want to test.

package org.kodejava.lang;

import java.util.Scanner;

import static java.lang.System.in;

public class StringStartsWithExample {
    private static final Scanner scanner = new Scanner(in);

    public static void main(String[] args) {
        String text = "The quick brown fox jumps over the lazy dog";
        System.out.println("Text: " + text);

        System.out.print("Type the prefix to test: ");
        String search = scanner.nextLine();

        // Check if the text start with the search text.
        if (text.startsWith(search)) {
            System.out.println("Yes, the fox is the quick one");
        } else {
            System.out.println("The fox is a slow fox");
        }
    }
}

The code snippet print the following output:

Text: The quick brown fox jumps over the lazy dog
Type the prefix to test: The quick brown fox
Yes, the fox is the quick one

How do I check a string ends with a specific word?

By Wayan in Core API, Lang Package 0 Comment

The String.endsWith() method can be used to check if a string ends with a specific word. It will return a boolean true if the suffix is found at the end of the string object.

In this example we will start the code by creating a class called StringEndsWithExample. This class has a standard main() method that makes the class executable. In the main() method we create a string variable called str and assign a text to it.

On the following line you can see an if conditional statement to check it the str string ends with "lazy dog". If it ends with that words then the corresponding block in the if statement will be executed.

package org.kodejava.lang;

public class StringEndsWithExample {
    public static void main(String[] args) {
        String str = "The quick brown fox jumps over the lazy dog";

        // well, does the fox jumps over a lazy dog?
        if (str.endsWith("lazy dog")) {
            System.out.println("The dog is a lazy dog");
        } else {
            System.out.println("Good dog!");
        }

        // Ends with empty string.
        if (str.endsWith("")) {
            System.out.println("true");
        }

        // Ends with the same string.
        if (str.endsWith(str)) {
            System.out.println("true");
        }
    }
}

Another thing that you need to know is that the endsWith() method will return true if you pass in an empty string or another string that is equals to this string as the argument. This method is also case-sensitive.

When you run the code snippet above you can see the following lines printed out:

The dog is a lazy dog
true
true

How do I know the length of a string?

By Wayan in Core API, Lang Package 0 Comment

To get the length of a String object we can use the String.length() method. This method returns the string length as an int value. The length of the string is equals to the length of the sequence of characters represented by this string object, and it is equals to the number of 16-bit Unicode characters in the string.

The following code snippet shows how to use the length() method. We create a string object name and assign a text to it. Call the method and store the length returned to the length variable and print it out. We can also use the length() method to check if the string is empty by checking if the length is equals to zero.

package org.kodejava.lang;

public class StringLengthExample {
    public static void main(String[] args) {
        String name = "Kode Java - Learn Java by Examples";

        // Get the length of the string specified by the name
        // variable.
        int length = name.length();
        System.out.println("Length = " + length);

        // Set name to an empty string and test if it is empty.
        name = "";
        length = name.length();
        if (length == 0) {
            System.out.println("The string is empty!");
        }
    }
}

If you run this code you will get the following output:

Length = 34
The string is empty!

How do I check string for equality?

By Wayan in Core API, Lang Package 0 Comment

To compare strings for their content equality we must use the String.equals() method. This method ensures that it will compare the content of both string instead of the object reference of the both string. This method returns true if both string in comparison have the same content.

Do not, never, use the == operator to compare string for its content. The == operator check for object reference equality, it returns true only if both objects point to the same reference. This operator returns false if the object doesn’t point to the same references.

package org.kodejava.lang;

public class StringEquals {
    public static void main(String[] args) {
        String s1 = "Hello World";
        String s2 = new String("Hello World");

        // This is good!
        if (s1.equals(s2)) {
            System.out.println("1. Both strings are equals.");
        } else {
            System.out.println("1. Both strings are not equals.");
        }

        // This is bad!
        if (s1 == s2) {
            System.out.println("2. Both strings are equals.");
        } else {
            System.out.println("2. Both strings are not equals.");
        }
    }
}

In the example above we deliberately create an instance of s2 string using the new operator to make sure that we have a different object reference. When you run the program it will give you the following result:

1. Both strings are equals.
2. Both strings are not equals.