[JAVA-621] Added missing code

* https://www.baeldung.com/java-type-erasure introduction code

* https://www.baeldung.com/java-interfaces Electronic interface and Computer class

* https://www.baeldung.com/java-abstract-class BoardGame and Checkers classes

* https://www.baeldung.com/java-hashcode different ways of implementing hashCode

* https://www.baeldung.com/java-inheritance-composition ComputerBuilder and StandardComputerBuilder classes

* https://www.baeldung.com/java-equals-hashcode-contracts Renamed method to match article

* https://www.baeldung.com/java-static Renamed class to match article

* https://www.baeldung.com/java-nested-classes Renamed class to match article

core-java-modules/core-java-lang-oop-inheritance/src/main/java/com/baeldung/abstractclasses/overview/BoardGame.java

@@ -0,0 +1,9 @@
+package com.baeldung.abstractclasses.overview;
+
+public abstract class BoardGame {
+    //... field declarations, constructors
+
+    public abstract void play();
+
+    //... concrete methods
+}

core-java-modules/core-java-lang-oop-inheritance/src/main/java/com/baeldung/abstractclasses/overview/Checkers.java

@@ -0,0 +1,8 @@
+package com.baeldung.abstractclasses.overview;
+
+public class Checkers extends BoardGame {
+    @Override
+    public void play() {
+        //... implementation
+    }
+}

core-java-modules/core-java-lang-oop-inheritance/src/main/java/com/baeldung/interfaces/Computer.java

@@ -0,0 +1,9 @@
+package com.baeldung.interfaces;
+
+public class Computer implements Electronic {
+
+    @Override
+    public int getElectricityUse() {
+        return 1000;
+    }
+}

core-java-modules/core-java-lang-oop-inheritance/src/main/java/com/baeldung/interfaces/Electronic.java

@@ -0,0 +1,22 @@
+package com.baeldung.interfaces;
+
+public interface Electronic {
+    // Constant variable
+    String LED = "LED";
+
+    // Abstract method
+    int getElectricityUse();
+
+    // Static method
+    static boolean isEnergyEfficient(String electtronicType) {
+        if (electtronicType.equals(LED)) {
+            return true;
+        }
+        return false;
+    }
+
+    //Default method
+    default void printDescription() {
+        System.out.println("Electronic Description");
+    }
+}