Setting breakpoints You can set breakpoints on any line of code, including the main method, the printAge method, and the for loops.
Stepping through code Once a breakpoint is hit, you can step through the code one line at a time using various options in the debugger, such as "Step Over" and "Step Into".
Viewing variables While stepping through the code, you can view the values of variables, including local variables, instance variables, and array elements.
Changing variables You can change the values of variables while debugging to see how the code behaves with different inputs.
Evaluating expressions You can evaluate expressions while debugging to see their results. For example, you could evaluate x + y to see the value of the sum variable.
Debugging conditional statements You can step through conditional statements and see how the code behaves depending on the condition.
Debugging loops You can step through loops and see how the code behaves as it iterates.
Debugging arrays You can step through arrays and see how the code accesses and modifies their elements.
By using a debugger to step through this class, you can see all of these features in action and gain a better understanding of how the code works.
Two methods called foo and bar are used to demonstrate the call stack feature. Here's how it works:
When the main method is called, it executes as usual until it gets to the bottom, where we've added a call to foo.
When foo is called, it starts executing, and immediately calls bar.
When bar is called, it starts executing, and creates a new stack frame on top of the existing stack frame for foo.
When bar is finished executing, it returns a value back to foo, and the stack frame for bar is removed.
When foo is finished executing, it returns control back to main, and the stack frame for foo is removed.
Finally, main finishes executing and the program terminates.
By stepping through the code and examining the call stack, you can see how each method call adds a new frame to the stack, and how the frames are removed when each method completes its execution. This can be a valuable tool for understanding how your code works and diagnosing issues that may arise.
In this code, we're demonstrating how to use jvisualvm to monitor a Java application's memory usage. Specifically, we're creating an ArrayList and adding one million String elements to it. Then we're adding a delay of 30 seconds to allow us to inspect the application with jvisualvm.By running jvisualvm and monitoring the Java process running our application, we can see how much memory is being used by the ArrayList. This is useful for identifying potential memory leaks or other performance issues that may be impacting the performance of our Java application.
To use jvisualvm to monitor the memory usage of our Java application, follow these steps:
Compile the code and run the program in a console or terminal window. Open jvisualvm from the command line by typing jvisualvm and pressing Enter. In jvisualvm, locate the running Java process for your application under the "Local" tab.
Double-click on the process to open the monitoring tab for that process. Select the "Memory" tab to view memory usage metrics for the Java process. Use the various graphs and metrics displayed on the "Memory" tab to analyze the memory usage of your Java application.
Some of the memory usage metrics that you might want to pay attention to include:
Heap memory usage This is the amount of memory being used by the Java heap, which stores objects and data used by the application.
Non-heap memory usage This is the amount of memory being used by non-heap memory areas, such as the Java stack or native memory.
Total memory usage This is the total amount of memory being used by the Java process, including both heap and non-heap memory.
Garbage collection activity This metric shows how often garbage collection is occurring, which can impact the performance of your Java application.
By using jvisualvm to monitor the memory usage of your Java applications, you can identify potential memory leaks or other performance issues that may be impacting the performance of your application.
Let's go through each feature one by one:
References We create two objects (firstObject and secondObject) and assign them to variables. We then assign secondObject to firstObject, creating a reference. This means that both variables now point to the same object in memory. We can inspect these references in jvisualvm by using the "Monitor" tab and looking at the "Heap Dump" and "Classes" sections.
Call Stack We create an object (thirdObject) inside a method (method()) and then let it go out of scope by returning from the method. We can inspect the call stack in jvisualvm by using the "Threads" tab and looking at the stack trace for the method() method.
Marking for Garbage Collection We create an object (secondObject) inside a code block and then let it go out of scope by exiting the code block. We then call System.gc() to trigger garbage collection explicitly. We can inspect the memory usage in jvisualvm by using the "Monitor" tab and looking at the "Memory" and "GC" sections. We can see that the memory usage drops after garbage collection occurs and that the secondObject object is no longer present in memory.
By understanding how to use jvisualvm to monitor references, call stacks, and garbage collection, we can optimize our Java applications and ensure that they use memory efficiently.
Here are some of the most frequently discussed exceptions in Java:NullPointerException This exception occurs when you try to use a null reference or variable in a way that requires a non-null value.
ClassNotFoundException This exception occurs when the class loader is unable to find the requested class at runtime.
IOException This exception occurs when an I/O operation (such as reading or writing a file) fails or is interrupted.
ArithmeticException This exception occurs when an arithmetic operation (such as division by zero) fails or is invalid.
ArrayIndexOutOfBoundsException This exception occurs when you try to access an array element that doesn't exist (i.e., an index that is less than zero or greater than or equal to the length of the array).
IllegalArgumentException This exception occurs when you pass an invalid argument to a method (such as a negative number where a positive one is expected).
ConcurrentModificationException This exception occurs when you try to modify a collection (such as a list or map) while it is being iterated over.
ClassCastException This exception occurs when you try to cast an object to a type that it is not (i.e., a type that is not a subclass or superclass of the object's actual type).
OutOfMemoryError This error occurs when the Java Virtual Machine (JVM) runs out of memory and is unable to allocate more.
StackOverflowError This error occurs when the call stack becomes too deep (i.e., there are too many nested method calls) and runs out of space.
These are just a few examples of the many exceptions that can occur in a Java program. Understanding these common exceptions and how to handle them can help you write more robust and reliable code.
This program intentionally throws each of the 10 exceptions discussed earlier, demonstrating how they can occur and providing an opportunity to handle them appropriately.
Refactoring refers to the process of improving the structure and organization of existing code without changing its external behavior. Here are some common refactoring features that developers often use:Extract Method This feature is used to break down large, complex methods into smaller, more focused ones. This makes the code more modular, easier to understand, and easier to maintain.
Rename This feature is used to change the name of variables, methods, classes, or other identifiers in the codebase. Renaming identifiers can improve code clarity and readability.
Extract Variable This feature is used to assign intermediate values to variables to make complex expressions more readable. Extracting variables can make the code easier to understand and can also improve performance.
Inline Variable This feature is used to replace variables with their values to simplify the code. This can make the code more readable and can also improve performance.
Extract Interface This feature is used to define common methods and properties as an interface, which can be implemented by multiple classes. Extracting an interface can improve the design of the code and make it more flexible.
Extract Class This feature is used to move fields and methods from one class to another to improve the design of the code. Extracting a class can make the code more modular and easier to maintain.
Replace Magic Number with Symbolic Constant This feature is used to replace hard-coded values in the code with named constants. Using named constants can make the code more readable and easier to maintain.
Encapsulate Field This feature is used to hide internal details of a class by providing accessor methods. Encapsulating fields can make the code more maintainable and easier to change.
Introduce Parameter Object This feature is used to group related method parameters into a single object. This can make the code more modular and easier to understand.
Introduce Null Object This feature is used to eliminate null checks by providing a null object that can be used in place of a null value. Introducing a null object can make the code more maintainable and easier to understand.
These are just a few of the many refactoring features that developers use to improve code quality. By using refactoring techniques, developers can make their code more modular, maintainable, and flexible.
This code demonstrates the following concepts:
JUnit fixtures The @Before annotation is used to create an instance of Calculator before each test method is run. This ensures that the test methods are executed in isolation and don't interfere with each other.
Test data The INPUT1, INPUT2, EXPECTED_SUM, and EXPECTED_DIFFERENCE constants define the inputs and expected outputs for the testAdd and testSubtract methods.
Mocking The @Mock annotation is used to create a mock instance of DataService. The when method is used to set up the mock to return test data when its getData method is called. The calculator instance is then injected with the mock using its setDataService method. Finally, the assertEquals method is used to verify that the calculateAverage method of calculator returns the expected value of 10.0. This isolates the unit under test (Calculator) from its dependencies (DataService) and ensures that it behaves as expected.
In this example, we have intentionally included some issues that PMD can detect:
Unused variables Both count and str are declared but never used, which PMD can detect.
Complex condition The if statement has a complex condition with multiple operators, which can be hard to read and maintain.
Inefficient use of array The for loop uses the length of the array to iterate, which can be inefficient compared to using an enhanced for loop.
SQL injection vulnerability The input variable is not validated before being used in a SQL query, which can allow attackers to execute arbitrary SQL commands.
Use of deprecated methods The Date constructor used in the bar() method is deprecated, which can lead to compatibility issues and reduced functionality.
By running PMD on this code, you can detect these issues and take steps to fix them, improving the quality and security of your code.
And here are some explanations of the bugs that are triggered in this code:
NP_NULL_ON_SOME_PATH_FROM_RETURN_VALUE: This bug is triggered when a null value is returned from a method, and the code does not check for null before accessing a method or field on the returned value. In this case, the list variable contains a null value, which will cause a NullPointerException to be thrown when the loop tries to access the toUpperCase() method on the null value.
NP_NULL_PARAM_DEREF_ALL_TARGETS_DANGEROUS: This bug is triggered when a null value is passed as a parameter to a method, and the method dereferences the parameter without first checking for null. In this case, the loop is iterating over the list variable, which contains a null value, and passing that null value to the toUpperCase() method, which will cause a NullPointerException to be thrown.
DM_NUMBER_CTOR: This bug is triggered when the constructor of the java.lang.Integer or java.lang.Long class is called with a value that can be represented as a constant expression. In this case, the i variable is initialized with the value 10, which is a constant expression that could be replaced with Integer.valueOf(10) for better performance.