[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-patterns/src/main/java/com/baeldung/inheritancecomposition/model/ComputerBuilder.java

@@ -0,0 +1,13 @@
+package com.baeldung.inheritancecomposition.model;
+
+public abstract class ComputerBuilder {
+
+    public final void buildComputer() {
+        addProcessor();
+        addMemory();
+    }
+
+    public abstract void addProcessor();
+
+    public abstract void addMemory();
+}

core-java-modules/core-java-lang-oop-patterns/src/main/java/com/baeldung/inheritancecomposition/model/StandardComputerBuilder.java

@@ -0,0 +1,13 @@
+package com.baeldung.inheritancecomposition.model;
+
+public class StandardComputerBuilder extends ComputerBuilder {
+    @Override
+    public void addProcessor() {
+        // method implementation
+    }
+
+    @Override
+    public void addMemory() {
+        // method implementation
+    }
+}