Oracle Tutoring, Campbell River, BC
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The java command, in Windows, anyway, seems to like a space between the directory and the filename you are running.
java -cp f:/dir1/dir2 program_file
not
java -cp f:/dir1/dir2/program_file
Source:
Jack of Oracle Tutoring by Jack and Diane, Campbell River, BC.
In Java, when you override a method, you either
.
Case 1, above, refers to the situation of writing a subclass of an abstract class, or implementing an interface: in either case, a method with undefined functionality must (typically) be defined, aka overridden.
Case 2 often refers to the situation of writing a subclass that alters the performance of an inherited method. However, an instance of a class can have a method overridden as well.
When overriding a method, its return type, name, and parameter list (including types) must be identical to its original ones [from the abstract class, interface, or base class].
@Override can be used at the beginning of the line where the override is declared. For example:
@Override public void method_x(String param0, int param1,….){
This may give the compiler extra opportunity to check the validity of the override, as well as improve the code’s human readability.
Source:
Jack of Oracle Tutoring by Jack and Diane, Campbell River, BC.
The message
error: non-static method MethodX() cannot be referenced from a static context
has a couple of causes I know:
Jack of Oracle Tutoring by Jack and Diane, Campbell River, BC.
Let’s imagine you write a subclass. Furthermore, let’s imagine the parent class (the superclass) has a detailed constructor. Can you assume that the subclass will simply inherit the parent’s constructor and default to its use? Apparently not.
My situation: I wrote a subclass of an abstract class that, while abstract, has a well-defined constructor. In the subclass I just defined the abstract functions from the parent. The compiler protested, citing a mismatch between the parameters required for the constructor versus those given. I looked up the problem.
Apparently, to a subclass, the compiler provides a default constructor with no arguments.
To instill the parent’s constructor in the subclass, one can declare a constructor in the subclass with the same parameters as those of the superclass’ constructor, then call super(parameter list).
Let’s imagine a parent class PC like so:
class PC{
String name;
String color;
}
Now, the subclass SC:
class SC extends PC{
}
From my observation, the code in the blue rectangles above is what will make the subclass take on the parent’s constructor.
Source:
Jack of Oracle Tutoring by Jack and Diane, Campbell River, BC.
I’m not trained in Java, but do write experimental programs with it. Here are some ideas about the import statement, as I understand it:
Herein, it might be helpful to think of a package as a directory.
The Java statement
import pckg0.member0;
gives access to the definition of member0, as needed for the program. In this context, member0 is a single, specific class found in the package pckg0.
The statement
import pckg0.*;
gives access to all “single” classes contained in pckg0, but not to those of its subdirectories. If pckg0 contains the package pckg00, which contains the class mem01, then the statement
import pckg0.pckg00.mem01;
provides access to it.
The statement
import pckg0.pckg00.*;
provides access to all single classes in pckg00.
Therefore, the two import statements, one atop the other,
import pckg0.*;
import pckg0.pck00.*;
can sensibly occur together at the top of a Java program.
HTH:)
Source:
McGrath, Mike. Java in Easy Steps, 5th ed. Warwickshire: Easy Steps Limited, 2014.
Jack of Oracle Tutoring by Jack and Diane, Campbell River, BC.
Null, it seems, is often mentioned in computer science. Perhaps it’s used a little loosely: it might refer to the value read when there’s nothing there, or the value of a variable that doesn’t exist.
In JavaScript, in my experience:
For a possible reason why a variable set as null is of type object, see my post here about Java and null. JavaScript isn’t the same as Java, but the two languages may relate to null with some similarity.
Source:
Jack of Oracle Tutoring by Jack and Diane, Campbell River, BC.
I don’t write many big programs; rather, I’ll often write a page or so of code (with lots of white space) to test a utility from a given language (lately it’s been Java). Such a program is usually run from the command line.
Some programs end up “hanging”; therefore, I like to know how to stop a program before I run it. If the program really gets out of hand (and assuming it’s running from the command line), there are two ways I know to stop it:
The Task Manager is easily started by right-clicking the taskbar, then clicking Start Task Manager. There are several tabs to choose from; the Applications one will show the Command Prompt. If the program is running from there, just click it, then End Task at the bottom.
The two methods work equally well on my system:)
Source:
Jack of Oracle Tutoring by Jack and Diane, Campbell River, BC.
Sometimes, reading Java code, you encounter null being passed as an argument aka parameter:
method0(null);
Apparently, when a method expects an object parameter, but doesn’t (necessarily) use it, you can pass it null to satisfy its need for a parameter. A very simple example might be as follows:
method0(Object obj0){
System.out.println(“Hello! I’m here!”);
}
Above, the method method0 requires a parameter of type Object. Although method0 doesn’t use that parameter, the compiler will protest if method0 is called without it. Calling method0 with null as the parameter will apparently satisfy the compiler, which is why you might see
method0(null);
in Java code examples.
I’ll talk more about this in future posts.
Source:
Jack of Oracle Tutoring by Jack and Diane, Campbell River, BC.