Add nullability concept (#2584)

* Nullability concept added

* Updating error message

* Updating config.json

* Linting issues and prerequisites updated in config.json

* Addressing review comments

* Removing trailing space

Fixes #2553

Author: smcg468

concepts/nullability/.meta/config.json

@@ -0,0 +1,7 @@
+{
+  "blurb": "In Java, the null literal is used to denote the absence of a value.",
+  "authors": [
+    "smcg468"
+  ],
+  "contributors": []
+}

concepts/nullability/about.md

@@ -0,0 +1,53 @@
+# About
+
+In Java, the [`null` literal][null-keyword] is used to denote the absence of a value.
+
+[Primitive variables][primitive-data-types] in java all have a default value and therefore can never be `null`.
+By convention, they start with a lowercase letter e.g `int`
+
+[Reference variables][reference-data-types] contain the memory address of an object and can have a value of null.
+These variables usually start with an uppercase e.g `String`
+
+Attempting to assign a primitive variable a value of `null` will result in a compile time error as the variable always holds
+a default value of the type assigned.
+
+```java
+//Throws compile time error stating the required type is int, but null was provided
+int number = null;
+```
+
+Assigning a reference variable a `null` value will not result in a compile time error as reference variables are nullable.
+
+```java
+//No error will occur as the String variable str is nullable
+String str = null;
+```
+
+Whilst accessing a reference variable which has a value of `null` will compile fine, it will result in a `NullPointerException` being thrown at runtime.
+
+```java
+int[] arr = null;
+
+// Throws NullPointerException at runtime
+arr.Length;
+```
+
+A [`NullPointerException` is thrown][null-pointer-exception] when trying to access a reference variable which is null but requires an object.
+
+To safely work with nullable values, one should check if they are `null` before working with them which can be done using [equality operators][equality-operators] such as `==` or `!=`:
+
+```java
+int[] arr = null;
+
+if(arr != null) {
+    System.out.println(arr.length);
+} else {
+    //Perform an alternate action when arr is null
+}
+```
+
+[null-keyword]: https://docs.oracle.com/javase/specs/jls/se7/html/jls-3.html#jls-3.10.7
+[primitive-data-types]: https://docs.oracle.com/javase/tutorial/java/nutsandbolts/datatypes.html
+[reference-data-types]: https://docs.oracle.com/javase/specs/jls/se7/html/jls-4.html#jls-4.3
+[null-pointer-exception]: https://docs.oracle.com/javase/8/docs/api/java/lang/NullPointerException.html
+[equality-operators]: https://docs.oracle.com/javase/tutorial/java/nutsandbolts/op2.html

concepts/nullability/introduction.md

@@ -0,0 +1,24 @@
+# Introduction
+
+In Java, the `null` literal is used to denote the absence of a value.
+
+Primitive data types in java all have a default value and therefore can never be `null`.
+By convention, they start with a lowercase letter e.g `int`
+
+Reference types contain the memory address of an object can have a value of null.
+These variables usually start with an uppercase e.g `String`
+
+Attempting to assign a primitive variable a value of `null` will result in a compile time error as the variable always holds
+a primitive value of the type assigned.
+
+```java
+//Throws compile time error stating the required type is int, but null was provided
+int number = null;
+```
+
+Assigning a reference variable a `null` value will not result in a compile time error as reference variables are nullable.
+
+```java
+//No error will occur as the String variable str is nullable
+String str = null;
+```

concepts/nullability/links.json

@@ -0,0 +1,22 @@
+[
+  {
+    "url": "https://docs.oracle.com/javase/specs/jls/se7/html/jls-3.html#jls-3.10.7",
+    "description": "null-keyword"
+  },
+  {
+    "url": "https://docs.oracle.com/javase/tutorial/java/nutsandbolts/datatypes.html",
+    "description": "primitive-data-types"
+  },
+  {
+    "url": "https://docs.oracle.com/javase/specs/jls/se7/html/jls-4.html#jls-4.3",
+    "description": "reference-data-types"
+  },
+  {
+    "url": "https://docs.oracle.com/javase/8/docs/api/java/lang/NullPointerException.html",
+    "description": "null-pointer-exception"
+  },
+  {
+    "url": "https://docs.oracle.com/javase/tutorial/java/nutsandbolts/op2.html",
+    "description": "equality-operators"
+  }
+]

config.json

@@ -234,6 +234,18 @@
           "switch-statement",
           "constructors"
         ]
+      },
+      {
+        "slug": "tim-from-marketing",
+        "name": "Tim from Marketing",
+        "uuid": "28bd20c5-4fdd-4660-9225-54f24aae24e4",
+        "concepts": [
+          "nullability"
+        ],
+        "prerequisites": [
+          "if-else-statements",
+          "strings"
+        ]
       }
     ],
     "practice": [
@@ -2088,6 +2100,11 @@
       "slug": "lists",
       "name": "Lists"
     },
+    {
+      "uuid": "0718bff1-25ad-42bb-860d-1b0834beb9fc",
+      "slug": "nullability",
+      "name": "Nullability"
+    },
     {
       "uuid": "58529dab-0ef2-4943-ac12-a98ca79b922b",
       "slug": "numbers",

exercises/concept/tim-from-marketing/.docs/hints.md

@@ -0,0 +1,20 @@
+# Hints
+
+## 1. Print a badge for an employee
+
+- [String interpolation][string-interpolation] can be used to create description strings.
+- There is a [built-in method to convert a string to uppercase][to-upper-case].
+
+## 2. Print a badge for a new employee
+
+- You should check if the ID is `null` before using it.
+- You can use the [equality operators][equality-operators] to compare the value to `null`
+
+## 3. Print a badge for the owner
+
+- You should check if the department is `null` before using it.
+- You can use the [equality operators][equality-operators] to compare the value to `null`
+
+[string-interpolation]: https://www.baeldung.com/java-string-interpolation
+[to-upper-case]:https://docs.oracle.com/javase/8/docs/api/java/lang/String.html#toUpperCase--
+[equality-operators]: https://docs.oracle.com/cd/E21764_01/apirefs.1111/e17787/com/sigmadynamics/util/Null.html#isNull_java_lang_String_

exercises/concept/tim-from-marketing/.docs/instructions.md

@@ -0,0 +1,47 @@
+# Instructions
+
+In this exercise you'll be writing code to print name badges for factory employees.
+
+## 1. Print a badge for an employee
+
+Employees have an ID, name and department name. Employee badge labels are formatted as follows: `"[id] - name - DEPARTMENT"`.
+Implement the `Badge.print()` method to return an employee's badge label:
+
+```java
+Badge badge = new Badge();
+badge.print(734, "Ernest Johnny Payne", "Strategic Communication");
+// => "[734] - Ernest Johnny Payne - STRATEGIC COMMUNICATION"
+```
+
+Note that the department should be uppercased on the label.
+
+## 2. Print a badge for a new employee
+
+Due to a quirk in the computer system, new employees occasionally don't yet have an ID when they start working at the factory.
+As badges are required, they will receive a temporary badge without the ID prefix. Modify the `Badge.print()` method to support new employees that don't yet have an ID:
+
+```java
+Badge badge = new Badge();
+Badge.print(null, "Jane Johnson", "Procurement");
+// => "Jane Johnson - PROCUREMENT"
+```
+
+## 3. Print a badge for the owner
+
+Even the factory's owner has to wear a badge at all times.
+However, an owner does not have a department. In this case, the label should print `"OWNER"` instead of the department name.
+Modify the `Badge.print()` method to print a label for the owner:
+
+```java
+Badge badge = new Badge();
+badge.print(254, "Charlotte Hale", null);
+// => "[254] - Charlotte Hale - OWNER"
+```
+
+Note that it is possible for the owner to also be a new employee:
+
+```java
+Badge badge = new Badge();
+badge.print(null, "Charlotte Hale", null);
+// => "Charlotte Hale - OWNER"
+```

exercises/concept/tim-from-marketing/.docs/introduction.md

@@ -0,0 +1,26 @@
+# Introduction
+
+## Nullability
+
+In Java, the `null` literal is used to denote the absence of a value.
+
+Primitive data types in java all have a default value and therefore can never be `null`.
+By convention, they start with a lowercase letter e.g `int`
+
+Reference types contain the memory address of an object can
+have a value of null. These variables usually start with an uppercase e.g `String`
+
+Attempting to assign a primitive variable a value of `null` will result in a compile time error as the variable always holds
+a primitive value of the type assigned.
+
+```java
+//Throws compile time error stating the required type is int, but null was provided
+int number = null;
+```
+
+Assigning a reference variable a `null` value will not result in a compile time error as reference variables are nullable.
+
+```java
+//No error will occur as the String variable str is nullable
+String str = null;
+```

exercises/concept/tim-from-marketing/.docs/introduction.md.tpl

@@ -0,0 +1,5 @@
+# Introduction
+
+## Nullability
+
+%{concept:nullability}

exercises/concept/tim-from-marketing/.meta/config.json

@@ -0,0 +1,23 @@
+{
+  "authors": [
+    "smcg468"
+  ],
+  "files": {
+    "solution": [
+      "src/main/java/Badge.java"
+    ],
+    "test": [
+      "src/test/java/BadgeTest.java"
+    ],
+    "exemplar": [
+      ".meta/src/reference/java/Badge.java"
+    ],
+    "invalidator": [
+      "build.gradle"
+    ]
+  },
+  "forked_from": [
+    "csharp/tim-from-marketing"
+  ],
+  "blurb": "Learn about the null literal and nullable variables in java by printing name badges."
+}

exercises/concept/tim-from-marketing/.meta/design.md

@@ -0,0 +1,20 @@
+# Design
+
+## Learning objectives
+
+- Know of the existence of the `null` literal.
+- Know what a `NullPointerException` is and when it is thrown.
+- Know how to compare a value to `null`.
+
+## Out of scope
+
+- `java.util.Optional`
+
+## Concepts
+
+- `nullability`
+
+## Prerequisites
+
+- `strings`: strings will be compared to null and basic methods from strings will be called.
+- `if-else-statements`: using a conditional statement.

exercises/concept/tim-from-marketing/.meta/src/reference/java/Badge.java

@@ -0,0 +1,20 @@
+public class Badge {
+
+    public String print(Integer id, String name, String department) {
+
+        String worksAt;
+
+        if (department == null) {
+            worksAt = "OWNER";
+        } else {
+            worksAt = department.toUpperCase();
+        }
+
+        if (id == null) {
+            return name + " - " + worksAt;
+        }
+
+        return "[" + id + "] - " + name + " - " + worksAt;
+    }
+
+}

exercises/concept/tim-from-marketing/build.gradle

@@ -0,0 +1,23 @@
+plugins {
+    id "java"
+}
+
+repositories {
+    mavenCentral()
+}
+
+dependencies {
+    testImplementation platform("org.junit:junit-bom:5.10.0")
+    testImplementation "org.junit.jupiter:junit-jupiter"
+    testImplementation "org.assertj:assertj-core:3.15.0"
+}
+
+test {
+    useJUnitPlatform()
+
+    testLogging {
+        exceptionFormat = "full"
+        showStandardStreams = true
+        events = ["passed", "failed", "skipped"]
+    }
+}

exercises/concept/tim-from-marketing/src/main/java/Badge.java

@@ -0,0 +1,5 @@
+class Badge {
+    public String print(Integer id, String name, String department) {
+        throw new UnsupportedOperationException("Please implement the (static) Badge.print() method");
+    }
+}

exercises/concept/tim-from-marketing/src/test/java/BadgeTest.java

@@ -0,0 +1,40 @@
+import org.junit.jupiter.api.DisplayName;
+import org.junit.jupiter.api.Tag;
+import org.junit.jupiter.api.Test;
+import static org.assertj.core.api.Assertions.assertThat;
+
+public class BadgeTest {
+
+    @Test
+    @Tag("task:1")
+    @DisplayName("Printing a badge for an employee")
+    public void labelForEmployee() {
+        Badge badge = new Badge();
+        assertThat(badge.print(17, "Ryder Herbert", "Marketing"))
+                .isEqualTo("[17] - Ryder Herbert - MARKETING");
+    }
+
+    @Test
+    @Tag("task:2")
+    @DisplayName("Printing a badge for a new employee")
+    public void labelForNewEmployee() {
+        Badge badge = new Badge();
+        assertThat(badge.print(null, "Bogdan Rosario", "Marketing")).isEqualTo("Bogdan Rosario - MARKETING");
+    }
+
+    @Test
+    @Tag("task:3")
+    @DisplayName("Printing a badge for the owner")
+    public void labelForOwner() {
+        Badge badge = new Badge();
+        assertThat(badge.print(59, "Julie Sokato", null)).isEqualTo("[59] - Julie Sokato - OWNER");
+    }
+
+    @Test
+    @Tag("task:3")
+    @DisplayName("Printing a badge for the owner who is a new employee")
+    public void labelForNewOwner() {
+        Badge badge = new Badge();
+        assertThat(badge.print(null, "Amare Osei", null)).isEqualTo("Amare Osei - OWNER");
+    }
+}

exercises/settings.gradle

@@ -17,6 +17,7 @@ include 'concept:football-match-reports'
 include 'concept:wizards-and-warriors'
 include 'concept:calculator-conundrum'
 include 'concept:logs-logs-logs'
+include 'concept:tim-from-marketing'
 
 // practice exercises
 include 'practice:accumulate'
@@ -150,3 +151,4 @@ include 'practice:wordy'
 include 'practice:yacht'
 include 'practice:zebra-puzzle'
 include 'practice:zipper'
+