Bael 1490 the checker framework and java pluggable type systems (#3584)

* BAEL-1490 First examples, Maven setup

* BAEL-1490 Each checker has its own source file

* BAEL-1490 Added checker for String.format

* BAEL-1490 The Checker Framework and Java Pluggable Type Systems

* Added comments, removed example that is probably too technical for a brad audience.

checker-plugin/src/main/java/com/baeldung/typechecker/FakeNumExample.java

@@ -0,0 +1,42 @@
+package com.baeldung.typechecker;
+
+import org.checkerframework.checker.fenum.qual.Fenum;
+
+//@SuppressWarnings("fenum:assignment.type.incompatible")
+public class FakeNumExample {
+
+    // Here we use some String constants to represents countries.
+    static final @Fenum("country") String ITALY = "IT";
+    static final @Fenum("country") String US = "US";
+    static final @Fenum("country") String UNITED_KINGDOM = "UK";
+
+    // Here we use other String constants to represent planets instead.
+    static final @Fenum("planet") String MARS = "Mars";
+    static final @Fenum("planet") String EARTH = "Earth";
+    static final @Fenum("planet") String VENUS = "Venus";
+
+    // Now we write this method and we want to be sure that
+    // the String parameter has a value of a country, not that is just a String.
+    void greetCountries(@Fenum("country") String country) {
+        System.out.println("Hello " + country);
+    }
+
+    // Similarly we're enforcing here that the provided
+    // parameter is a String that represent a planet.
+    void greetPlanets(@Fenum("planet") String planet) {
+        System.out.println("Hello " + planet);
+    }
+
+    public static void main(String[] args) {
+
+        FakeNumExample obj = new FakeNumExample();
+
+        // This will fail because we pass a planet-String to a method that
+        // accept a country-String.
+        obj.greetCountries(MARS);
+
+        // Here the opposite happens.
+        obj.greetPlanets(US);
+    }
+
+}

checker-plugin/src/main/java/com/baeldung/typechecker/FormatExample.java

@@ -0,0 +1,23 @@
+package com.baeldung.typechecker;
+
+public class FormatExample {
+
+    public static void main(String[] args) {
+
+        // Just enabling org.checkerframework.checker.formatter.FormatterChecker
+        // we can be sure at compile time that the format strings we pass to format()
+        // are correct. Normally we would have those errors raised just at runtime.
+        // All those formats are in fact wrong.
+
+        String.format("%y", 7);           // error: invalid format string
+
+        String.format("%d", "a string");  // error: invalid argument type for %d
+
+        String.format("%d %s", 7);        // error: missing argument for %s
+
+        String.format("%d", 7, 3);        // warning: unused argument 3
+
+        String.format("{0}", 7);          // warning: unused argument 7, because {0} is wrong syntax
+    }
+
+}

checker-plugin/src/main/java/com/baeldung/typechecker/KeyForExample.java

@@ -0,0 +1,31 @@
+package com.baeldung.typechecker;
+
+import org.checkerframework.checker.nullness.qual.KeyFor;
+
+import java.util.HashMap;
+import java.util.Map;
+
+public class KeyForExample {
+
+    private final Map<String, String> config = new HashMap<>();
+
+    KeyForExample() {
+
+        // Here we initialize a map to store
+        // some config data.
+        config.put("url", "http://1.2.3.4");
+        config.put("name", "foobaz");
+    }
+
+    public void dumpPort() {
+
+        // Here, we want to dump the port value stored in the
+        // config, so we declare that this key has to be
+        // present in the config map.
+        // Obviously that will fail because such key is not present
+        // in the map.
+        @KeyFor("config") String key = "port";
+        System.out.println( config.get(key) );
+    }
+
+}

checker-plugin/src/main/java/com/baeldung/typechecker/MonotonicNotNullExample.java

@@ -0,0 +1,28 @@
+package com.baeldung.typechecker;
+
+import org.checkerframework.checker.nullness.qual.MonotonicNonNull;
+
+import java.util.Date;
+
+public class MonotonicNotNullExample {
+
+    // The idea is we need this field to be to lazily initialized,
+    // so it starts as null but once it becomes not null
+    // it cannot return null.
+    // In these cases, we can use @MonotonicNonNull
+    @MonotonicNonNull private Date firstCall;
+
+    public Date getFirstCall() {
+        if (firstCall == null) {
+            firstCall = new Date();
+        }
+        return firstCall;
+    }
+
+    public void reset() {
+        // This is reported as error because
+        // we wrongly set the field back to null.
+        firstCall = null;
+    }
+
+}

checker-plugin/src/main/java/com/baeldung/typechecker/NonNullExample.java

@@ -0,0 +1,27 @@
+package com.baeldung.typechecker;
+
+import org.checkerframework.checker.nullness.qual.NonNull;
+import org.checkerframework.checker.nullness.qual.Nullable;
+
+public class NonNullExample {
+
+    // This method's parameter is annotated in order
+    // to tell the pluggable type system that it can never be null.
+    private static int countArgs(@NonNull String[] args) {
+        return args.length;
+    }
+
+    // This method's parameter is annotated in order
+    // to tell the pluggable type system that it may be null.
+    public static void main(@Nullable String[] args) {
+
+        // Here lies a potential error,
+        // because we pass a potential null reference to a method
+        // that does not accept nulls.
+        // The Checker Framework will spot this problem at compile time
+        // instead of runtime.
+        System.out.println(countArgs(args));
+
+    }
+
+}

checker-plugin/src/main/java/com/baeldung/typechecker/RegexExample.java

@@ -0,0 +1,18 @@
+package com.baeldung.typechecker;
+
+import org.checkerframework.checker.regex.qual.Regex;
+
+public class RegexExample {
+
+    // For some reason we want to be sure that this regex
+    // contains at least one capturing group.
+    // However, we do an error and we forgot to define such
+    // capturing group in it.
+    @Regex(1) private static String findNumbers = "\\d*";
+
+    public static void main(String[] args) {
+        String message = "My phone number is +3911223344.";
+        message.matches(findNumbers);
+    }
+
+}