visual basic barcode printing You must not return anything from a method with a void return type. in Java

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5. You must not return anything from a method with a void return type.
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public void bar() { return "this is it"; } // Not legal!!
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6. In a method with an object reference return type, you can return any object type that can be implicitly cast to the declared return type.
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public Animal getAnimal() { return new Horse(); // Assume Horse extends Animal } public Object getObject() { int[] nums = {1,2,3}; return nums; // Return an int array, which is still an object } public interface Chewable { } public class Gum implements Chewable { } public class TestChewable { // Method with an interface return type public Chewable getChewable {
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Legal Return Types (Exam Objective 1.4)
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return new Gum(); } }
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// Return interface implementer
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Watch for methods that declare an abstract class or interface return type, and know that any object that passes the IS-A test (in other words, would test true using the instanceof operator) can be returned from that method for example:
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public abstract class Animal { } public class Bear extends Animal { } public class Test { public Animal go() { return new Bear(); // OK, Bear "is-a" Animal } }
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Be sure you understand the rules for casting primitives. Take a look at the following:
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public short s = (short) (90 + 900000);
The preceding code compiles fine. But look at this variation:
public short s = (short) 90 + 900000; // Illegal!
By leaving off the parentheses around the arithmetic expression, the cast (short) applies only to the first number! So the compiler gives us
Test.java:4: possible loss of precision found : int required: short short s = (short) 90 + 900000; ^
Casting rules matter when returning values, so the following code would not compile,
public short foo() { return (short) 90 + 900000; }
but with parentheses around (90 + 900000), it compiles fine.
5: Object Orientation, Overloading and Overriding, Constructors, and Return Types
CERTIFICATION SUMMARY
Let s take a stroll through 5 and see where we ve been. You looked at how encapsulation can save you from being ripped to shreds by programmers whose code you could break if you change the way client code accesses your data. Protecting the instance variables (often by marking them private) and providing more accessible getter and setter methods represent the good OO practice of encapsulation, and support flexibility and maintainability by hiding your implementation details from other code. You learned that inheritance relationships are described using IS-A, as in Car IS-A Vehicle, and that the keyword extends is used to define IS-A relationships in Java:
class Car extends Vehicle
You also learned that reference relationships are described using HAS-A, as in Car HAS-A Engine. HAS-A relationships in Java often are defined by giving one class a reference to another, usually through instance variable declarations:
class Car extends Vehicle { private Engine eng; // Now Car has-a Engine, // and can thus invoke methods on it. }
We looked at the difference between overridden and overloaded methods, learning that an overridden method occurs when a subclass inherits a method from a superclass, but the subclass redefines it to add more specialized behavior. We learned that at runtime, the JVM will invoke the subclass version on an instance of a subclass, and the superclass version on an instance of the superclass. Remember that abstract methods must be overridden (technically abstract methods must be implemented, as opposed to overridden, since there really isn t anything to override in an abstract method, but who s counting ). We saw that overriding methods must keep the same argument list and return type as the overridden method, and that the access modifier can t be more restrictive. The overriding method also can t throw any new or broader checked exceptions that weren t declared in the overridden method. You also learned that the overridden method can be invoked using the syntax super.doSomething();.
Legal Return Types (Exam Objective 1.4)
Overloaded methods let you reuse the same method name in a class, but with different arguments (and optionally, a different return type). Whereas overriding methods must not change the argument list, overloaded methods must. But unlike overriding methods, overloaded methods are free to vary the return type, access modifier, and declared exceptions any way they like. We covered constructors in detail, learning that even if you don t provide a constructor for your class, the compiler will always insert one. The compiler-generated constructor is called the default constructor, and it is always a no-arg constructor with a no-arg call to super(). The default constructor will never be generated if there is even a single constructor in your class (and regardless of the arguments of that constructor), so if you need more than one constructor in your class and you want a no-arg constructor, you ll have to write it yourself. We also saw that constructors are not inherited, and that you can be confused by a method that has the same name as the class (which is legal). The return type is the giveaway that a method is not a constructor, since constructors do not have return types. We saw how all of the constructors in an object s inheritance tree will always be invoked when the object is instantiated using new. We also saw that constructors can be overloaded, which means defining constructors with different argument lists. A constructor can invoke another constructor of the same class using the keyword this(), as though the constructor were a method named this(). We saw that every constructor must have either this() or super() as the first statement. We also looked at method return types, and saw that you can declare any return type you like (assuming you have access to a class for an object reference return type), unless you re overriding a method. An overriding method must have the same return type as the overridden method of the superclass. We saw that while overriding methods must not change the return type, overloaded methods can (as long as they also change the argument list). Finally, you learned that it is legal to return any value or variable that can be implicitly converted to the declared return type. So, for example, a short can be returned when the return type is declared as an int. And a Horse reference can be returned when the return type is declared an Animal (assuming Horse extends Animal). And once again, you learned that the exam includes tricky questions designed largely to test your ability to recognize just how tricky the questions can be. If you took our advice about the margarita, you might want to review the following Two-Minute Drill again after you re sober.
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