Docker: Remove all the stopped containers

Execute below command to remove all the stopped containers.

docker rm $(docker ps -a -q)

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Javassist: Remove a method from an existing class

Follow below steps to remove a method from an existing class.

Step 1: Get ClassPool instance.

ClassPool classPool = ClassPool.getDefault();

Step 2: Get CtClass instace for the class that you want to remove a method.

CtClass testClass = classPool.get("com.sample.app.Test");

Step 3: Get CtMethod instance of the class that you want to remove.

CtMethod sayHelloMethod = testClass.getDeclaredMethod("sayHello");

Step 4: Use removeMethod function to remove the method.

testClass.removeMethod(sayHelloMethod);

Step 5: Publish byte code changes.

testClass.toClass();

Find below working application.

Test.java

package com.sample.app;

public class Test {
    
    public void sayHello() {
        System.out.println("Hello");
    }
    
    public void sayHi() {
        System.out.println("Hi");
    }

}

App.java

package com.sample.app;

import javassist.ClassPool;
import javassist.CtClass;
import javassist.CtMethod;

public class App {

    public static void main(String args[]) throws Exception {

        ClassPool classPool = ClassPool.getDefault();

        CtClass testClass = classPool.get("com.sample.app.Test");

        CtMethod sayHelloMethod = testClass.getDeclaredMethod("sayHello");
        testClass.removeMethod(sayHelloMethod);

        testClass.toClass();

        Test test = new Test();
        test.sayHi();
        test.sayHello();
    }
}

Run App.java, you will see 'NoSuchMethodError' in console.

Hi

Exception in thread "main" java.lang.NoSuchMethodError: com.sample.app.Test.sayHello()V

    at com.sample.app.App.main(App.java:22)

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Javassist: Remove CtClass object from ClassPool

If number of CtClass objects handled by ClassPool object are large, then you may end up in high menory memory consumption problems. To solve this, whenever you do not need CtClass objects, remove them from the pool explicitly.

How to remove a CtClass from ClassPool?

Using dtach method of CtClass, you can remove the CtClass object from pool.

Example

CtClass ctClass = ... ;

ctClass.detach();

App.java

package com.sample.app;

import javassist.ClassPool;
import javassist.CtClass;

public class App {

 public static void main(String args[]) throws Exception {

  ClassPool classPool = ClassPool.getDefault();
  classPool.makeClass("com.sample.app.model.Point");

  CtClass ctClass1 = classPool.get("com.sample.app.model.Point");

  System.out.println("Class Name : " + ctClass1.getName());

  System.out.println("Detaching the class from pool");
  ctClass1.detach();

  System.out.println("Retrieving the ctClass from pool");
  CtClass ctClass2 = classPool.get("com.sample.app.model.Point");

  System.out.println("ctClass2 : " + ctClass2);
 }
}

Run App.java, you will get below messages in console.

Class Name : com.sample.app.model.Point

Detaching the class from pool

Retrieving the ctClass from pool

Exception in thread "main" javassist.NotFoundException: com.sample.app.model.Point

         at javassist.ClassPool.get(ClassPool.java:430)

         at com.sample.app.App.main(App.java:21)

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Java: Remove first character of a string

Using ‘subString’ method, you can extract the string except the first character.

Example

public static String removeFirstChar(String str) {

         if (str == null || str.isEmpty()) {

                  throw new IllegalArgumentException("Input must not be null or empty");

         }

        

         return str.substring(1);

}

StringUtil.java

package com.smaple.app.utils;

public class StringUtil {

 public static String removeFirstChar(String str) {
  if (str == null || str.isEmpty()) {
   throw new IllegalArgumentException("Input must not be null or empty");
  }

  return str.substring(1);

 }
}

StringUtilTest.java

package com.sample.app.utils;

import static com.smaple.app.utils.StringUtil.removeFirstChar;
import static org.junit.Assert.assertEquals;

import org.junit.Test;

public class StringUtilTest {

 @Test(expected = IllegalArgumentException.class)
 public void inputIsNull() {
  String text = null;

  removeFirstChar(text);
 }

 @Test(expected = IllegalArgumentException.class)
 public void inputIsEmpty() {
  String text = "";

  removeFirstChar(text);
 }

 @Test
 public void inputLengthIs1() {
  String text = "a";
  String expected = "";
  
  String result = removeFirstChar(text);
  assertEquals(expected, result);
 }

 @Test
 public void inputLengthIsGreaterThan1() {
  String text = "hello";
  String expected = "ello";

  String result = removeFirstChar(text);

  assertEquals(expected, result);

 }
}

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How to Initialize Multiple Variables to same value in Java?

Extract digits from a string or text

TableSaw: Remove columns with missing values

‘table.removeColumnsWithMissingValues’ method remove the columns with missing values.

App.java

package com.sample.app;

import tech.tablesaw.api.IntColumn;
import tech.tablesaw.api.StringColumn;
import tech.tablesaw.api.Table;

public class App {

    public static void main(String args[]) {
        int[] empIds = { 1, 2, 3, 4, 5, 6, 7, 8, 9 };
        String[] firstNames = { "Hari", null, null, null, "Harini", "Lahari", "Rama", "Lakshman", "Sandya" };
        String[] lastNames = { "Krishna", "Gurram", "Maj", "Dev", "Gurram", "Ram", "Sen", "Grandi", "Neelam" };

        Table table = Table.create().addColumns(IntColumn.create("Employee Ids", empIds))
                .addColumns(StringColumn.create("FirstName", firstNames))
                .addColumns(StringColumn.create("LastName", lastNames));

        System.out.println(table.print());

        table.removeColumnsWithMissingValues();

        System.out.println("\n\n" + table.print());

    }
}

Output

Employee Ids  |  FirstName  |  LastName  |
-------------------------------------------
            1  |       Hari  |   Krishna  |
            2  |             |    Gurram  |
            3  |             |       Maj  |
            4  |             |       Dev  |
            5  |     Harini  |    Gurram  |
            6  |     Lahari  |       Ram  |
            7  |       Rama  |       Sen  |
            8  |   Lakshman  |    Grandi  |
            9  |     Sandya  |    Neelam  |


 Employee Ids  |  LastName  |
-----------------------------
            1  |   Krishna  |
            2  |    Gurram  |
            3  |       Maj  |
            4  |       Dev  |
            5  |    Gurram  |
            6  |       Ram  |
            7  |       Sen  |
            8  |    Grandi  |
            9  |    Neelam  |

As you see in the above example, Column FirstName has missing values for the rows 2, 3, and 4. When I call the method ‘removeColumnsWithMissingValues’ it removes the column FirstName.

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Table: Remove specific columns from a table

TableSaw provides below methods to remove columns from a table.

public Table removeColumns(Column<?>... columns)

public Relation removeColumns(int... columnIndexes)

public Relation removeColumns(String... columnName)

App.java

package com.sample.app;

import tech.tablesaw.api.IntColumn;
import tech.tablesaw.api.StringColumn;
import tech.tablesaw.api.Table;

public class App {

 public static void main(String args[]) {
  int[] empIds = { 1, 2, 3, 4, 5, 6, 7, 8, 9 };
  String[] firstNames = { "Hari", "Ram", "Chamu", "Sowmya", "Harini", "Lahari", "Rama", "Lakshman", "Sandya" };
  String[] lastNames = {"Krishna", "Gurram", "Maj", "Dev", "Gurram", "Ram", "Sen", "Grandi", "Neelam"};

  Table table = Table.create().addColumns(IntColumn.create("Employee Ids", empIds))
    .addColumns(StringColumn.create("FirstName", firstNames)).addColumns(StringColumn.create("LastName", lastNames));

 
  
  System.out.println(table.print());
  
  table.removeColumns("FirstName");
  System.out.println("\nRemoving the column FirstName\n");
  
  System.out.println(table.print());
  
 }
}

Output

Employee Ids  |  FirstName  |  LastName  |
-------------------------------------------
            1  |       Hari  |   Krishna  |
            2  |        Ram  |    Gurram  |
            3  |      Chamu  |       Maj  |
            4  |     Sowmya  |       Dev  |
            5  |     Harini  |    Gurram  |
            6  |     Lahari  |       Ram  |
            7  |       Rama  |       Sen  |
            8  |   Lakshman  |    Grandi  |
            9  |     Sandya  |    Neelam  |

Removing the column FirstName

 Employee Ids  |  LastName  |
-----------------------------
            1  |   Krishna  |
            2  |    Gurram  |
            3  |       Maj  |
            4  |       Dev  |
            5  |    Gurram  |
            6  |       Ram  |
            7  |       Sen  |
            8  |    Grandi  |
            9  |    Neelam  |

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