The stack is a linear data structure that is used to store the collection of objects. It is based on Last-In-First-Out (LIFO). Java collection framework provides many interfaces and classes to store the collection of objects. One of them is the Stack class that provides different operations such as push, pop, search, etc.
In this section, we will discuss the Java Stack class, its methods, and implement the stack data structure in a Java program. But before moving to the Java Stack class have a quick view of how the stack works.
Program For Stack In Java
The stack data structure has the two most important operations that are push and pop. The push operation inserts an element into the stack and pop operation removes an element from the top of the stack. Let's see how they work on stack.
In Java, Stack is a class that falls under the Collection framework that extends the Vector class. It also implements interfaces List, Collection, Iterable, Cloneable, Serializable. It represents the LIFO stack of objects. Before using the Stack class, we must import the java.util package. The stack class arranged in the Collections framework hierarchy, as shown below.
We can perform push, pop, peek and search operation on the stack. The Java Stack class provides mainly five methods to perform these operations. Along with this, it also provides all the methods of the Java Vector class.
The empty() method of the Stack class check the stack is empty or not. If the stack is empty, it returns true, else returns false. We can also use the isEmpty() method of the Vector class.
In the following example, we have created an instance of the Stack class. After that, we have invoked the empty() method two times. The first time it returns true because we have not pushed any element into the stack. After that, we have pushed elements into the stack. Again we have invoked the empty() method that returns false because the stack is not empty.
The method inserts an item onto the top of the stack. It works the same as the method addElement(item) method of the Vector class. It passes a parameter item to be pushed into the stack.
The method searches the object in the stack from the top. It parses a parameter that we want to search for. It returns the 1-based location of the object in the stack. Thes topmost object of the stack is considered at distance 1.
Suppose, o is an object in the stack that we want to search for. The method returns the distance from the top of the stack of the occurrence nearest the top of the stack. It uses equals() method to search an object in the stack.
Removes an item from the stack. The items are popped in the reversed order in which they are pushed. If the stack is empty, then it is said to be an Underflow condition.
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About this question - quite often I see a question come through where a novice programmer is "getting an error", and they simply paste their stack trace and some random block of code without understanding what the stack trace is or how they can use it. This question is intended as a reference for novice programmers who might need help understanding the value of a stack trace.
This is a very simple stack trace. If we start at the beginning of the list of "at ...", we can tell where our error happened. What we're looking for is the topmost method call that is part of our application. In this case, it's:
What's different about this one is the "Caused by". Sometimes exceptions will have multiple "Caused by" sections. For these, you typically want to find the "root cause", which will be one of the lowest "Caused by" sections in the stack trace. In our case, it's:
Like in previous examples, we should look at MyEntityService.java on line 59, because that's where this error originated (this one's a bit obvious what went wrong, since the SQLException states the error, but the debugging procedure is what we're after).
A stacktrace is a very helpful debugging tool. It shows the call stack (meaning, the stack of functions that were called up to that point) at the time an uncaught exception was thrown (or the time the stacktrace was generated manually). This is very useful because it doesn't only show you where the error happened, but also how the program ended up in that place of the code.This leads over to the next question:
Sometimes you can't make sure that you don't get an exception. For example, if you are using a network connection in your program, you cannot stop the computer from loosing it's internet connection (e.g. you can't stop the user from disconnecting the computer's network connection). In this case the network library will probably throw an exception. Now you should catch the exception and handle it. This means, in the example with the network connection, you should try to reopen the connection or notify the user or something like that. Also, whenever you use catch, always catch only the exception you want to catch, do not use broad catch statements like catch (Exception e) that would catch all exceptions. This is very important, because otherwise you might accidentally catch the wrong exception and react in the wrong way.
To add on to what Rob has mentioned. Setting break points in your application allows for the step-by-step processing of the stack. This enables the developer to use the debugger to see at what exact point the method is doing something that was unanticipated.
The above is the proper order to check for nulls, we start with the base object, dog in this case, and then begin walking down the tree of possibilities to make sure everything is valid before processing. If the order were reversed a NPE could potentially be thrown and our program would crash.
To understand the name: A stack trace is a a list of Exceptions( or you can say a list of "Cause by"), from the most surface Exception(e.g. Service Layer Exception) to the deepest one (e.g. Database Exception). Just like the reason we call it 'stack' is because stack is First in Last out (FILO), the deepest exception was happened in the very beginning, then a chain of exception was generated a series of consequences, the surface Exception was the last one happened in time, but we see it in the first place.
Key 1:A tricky and important thing here need to be understand is : the deepest cause may not be the "root cause", because if you write some "bad code", it may cause some exception underneath which is deeper than its layer. For example, a bad sql query may cause SQLServerException connection reset in the bottem instead of syndax error, which may just in the middle of the stack.
If you get a stack trace and want to trace the cause of the exception, a good start point in understanding it is to use the Java Stack Trace Console in Eclipse. If you use another IDE there may be a similar feature, but this answer is about Eclipse.
The most recent method call made (and the one that caused the exception) will be the top of the stack, which is the top line (excluding the error message text). In this case, that is the trimmedLength method. Going down the stack goes back in time. The second line is the method that calls the first line, etc.
Calling System.exit(0) (or any other value for that matter) causes the Java virtual machine to exit, terminating the current process. The parameter you pass will be the return value that the java process will return to the operating system. You can make this call from anywhere in your program - and the result will always be the same - JVM terminates. As this is simply calling a static method in System class, the compiler does not know what it will do - and hence does not complain about unreachable code.
return would cause the program to exit only if it's inside the main method of the main class being execute. If you try to put code after it, the compiler will complain about unreachable code, for example:
Because System.exit() is just another method to the compiler. It doesn't read ahead and figure out that the whole program will quit at that point (the JVM quits). Your OS or shell can read the integer that is passed back in the System.exit() method. It is standard for 0 to mean "program quit and everything went OK" and any other value to notify an error occurred. It is up to the developer to document these return values for any users.
return on the other hand is a reserved key word that the compiler knows well.return returns a value and ends the current function's run moving back up the stack to the function that invoked it (if any). In your code above it returns void as you have not supplied anything to return.
System.exit() terminates the JVM. Nothing after System.exit() is executed.Return is generally used for exiting a method. If the return type is void, then you could use return;But I don't think is a good practice to do it in the main method.You don't have to do anything for terminate a program, unless infinite loop or some strange other execution flows.
So return; does not really terminate your java program, it only terminates your java function (void). Because in your case the function is the main function of your application, return; will also terminate the application. But the return; in your example is useless, because the function will terminate directly after this return anyway... 2ff7e9595c
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