Day 3 break 2

Questions.txt

@@ -605,3 +605,28 @@ B) Exception at runtime
 C) Hello!, C-si: y is 300, P-i: y is 101, P-c: y is 401, C-i x: is 200 Goodbye!
 D) , P-si: x is 0, C-si: y is 300Hello!, P-i: y is 101, P-c: y is 401, C-i x: is 200 Goodbye!
 E) Hello!, P-si: x is 99, C-si: y is 300, P-i: y is 101, P-c: y is 100Goodbye!
+
+Q) Given:
+interface StudentCriterion {
+  boolean test(Student s);
+}
+
+and:
+class Student {
+  public int getGpa() { return 0; }
+  public List<String> getCourses() { return null; }
+}
+
+Which are legal (individually):
+A) StudentCriterion sc = Student s -> { return s.getGpa() > 3;};
+B) StudentCriterion sc = s -> { s.getGpa() > 3 };
+C) Object sc = s -> {return s.getGpa() > 3;};
+D) StudentCriterion sc = (s) -> s.getGpa() > 3;
+E) StudentCriterion sc = s -> s.getCourses().size();
+
+Q)   Which are legal individually:
+A) Function<String, String> fss = s -> { System.out.println(s); };
+B) Consumer<String> cs = s -> { System.out.println(s); };
+C) Supplier<String> ss = (s1, s2) -> s1 + s2;
+D) BiPredicate<String> bps = (String s) -> { return s.length() > 0; };
+E) ToIntFunction<String> tofs = s -> s.length();

src/main/java/lambdas/Demos.java

@@ -0,0 +1,76 @@
+package lambdas;
+
+import java.util.List;
+import java.util.function.ToIntFunction;
+
+interface StudentCriterion {
+  boolean test(Student s);
+}
+
+class Student {
+  public int getGpa() { return 0; }
+  public List<String> getCourses() { return null; }
+}
+
+class SmartStudent implements StudentCriterion {
+  @Override
+  public boolean test(Student s) {
+    return s.getGpa() > 3;
+  }
+}
+
+public class Demos {
+  public static void main(String[] args) {
+    StudentCriterion sc = new SmartStudent();
+    // lambda MUST implement an interface
+    // ** context MUST make that interface clear
+    // and that interface MUST have ONE abstract method
+    // we MUST ONLY desire to provide implementation of THAT ONE method
+    // compiler will:
+    // write the class using this method
+    // instantiate it :)
+//    StudentCriterion sc2 = (Student s) -> { return s.getGpa() > 3; };
+    // if the argument type is also inferrable, we can leave it out
+//    StudentCriterion sc2 = (s) -> { return s.getGpa() > 3; };
+    // sometimes, we want to annotate the formal parameter
+//    StudentCriterion sc2 = (@Deprecated Student s) -> { return s.getGpa() > 3; };
+    // this does NOT work (annotation goes with the TYPE not the formal param
+//    StudentCriterion sc2 = (@Deprecated s) -> { return s.getGpa() > 3; };
+//    StudentCriterion sc2 = (@Deprecated var s) -> { return s.getGpa() > 3; };
+//    StudentCriterion sc2 = (var s) -> { return s.getGpa() > 3; };
+    // BUT, you must choose:
+    // All formal params, with no type, with explicit type, or with var
+
+    // IF (but only if) you have a SINGLE formal param, with NO TYPE specifier
+    // we can leave off the parens (there's nothing to "group")
+//    StudentCriterion sc2 = s -> { return s.getGpa() > 3; };
+
+    // OFTEN, our method body simply returns an expression
+    // In which case we can leave out "clutter"
+    // curlies "group" stuff, but we have only one thing-- leave out
+    // return is unnecessary
+    // must do both changes at once :)
+//    StudentCriterion sc2 = s -> s.getGpa() > 3 ;
+//    StudentCriterion sc2 = s -> Boolean.valueOf(s.getGpa() > 3) ;
+    StudentCriterion sc2 = s -> {
+      Boolean b = null;
+      return b;
+    };
+
+    // CANNOT DO THIS!
+//    Object obj = (Student s) -> {
+//      return s.getGpa() > 3;
+//    };
+
+
+    // statement expression, value we can ignore
+//    boolean b = List.of().add(0);
+//    List.of().add(0);
+//    3 + 2;\
+
+    ToIntFunction<String> tofs = s -> Integer.valueOf(s.length());
+//    Long lng = 99; // Nope
+    Byte b = null;
+    long lng = b;
+  }
+}