Classic java interview questions_must ask for interns!

First, let’s talk about the difference between final, finally, and finalize.

final? Modifier (keyword) If a class is declared final, it means that it can no longer derive new subclasses and cannot be inherited as a parent class. Therefore a class cannot be declared both abstract and final. Declare variables or methods as final to ensure that they will not be changed during use. Variables declared as final must be given an initial value when declared, and can only be read in subsequent references and cannot be modified. Methods declared as final can also only be used and cannot be overloaded.

finally? Provide a finally block to perform any cleanup operations during exception handling. If an exception is thrown, the matching catch clause is executed and control passes to the finally block (if there is one).

finalize?Method name. Java technology allows the use of the finalize() method to do the necessary cleanup before the garbage collector clears the object from memory. This method is called by the garbage collector on this object when it determines that the object is not referenced. It is defined in the Object class, so all classes inherit it. Subclasses override the finalize() method to organize system resources or perform other cleanup work. The finalize() method is called on the object before the garbage collector deletes it.

Second, the difference between HashMap and Hashtable.

are all classes belonging to the Map interface, which implement mapping unique keys to specific values.

The HashMap class does not classify or sort. It allows a null key and multiple null values.

Hashtable is similar to HashMap, but does not allow null keys and null values. It is also slower than HashMap because it is synchronous.

Third, String s = new String("xyz"); How many String Objects are created?

Two objects, one is "xyx", the other Is the reference object s pointing to "xyx".

Fourth, what is the difference between sleep() and wait()? The favorite of threads

The sleep() method is a method to stop the thread for a period of time. After the sleep interval expires, the thread does not necessarily resume execution immediately. This is because at that moment, other threads may be running and are not scheduled to give up execution, unless (a) the "waking up" thread has a higher priority.

(b) The running thread is blocked for other reasons.

wait() is when threads interact. If the thread issues a wait() call to a synchronization object x, the thread will suspend execution and the called object will enter the waiting state until it is awakened or the waiting time expires.

Fifth, short s1 = 1; s1 = s1 1; What’s wrong? short s1 = 1; s1 = 1; What’s wrong?

short s1 = 1; s1 = s1 1; Wrong, s1 is of type short, s1 1 is of type int, and cannot be converted to short type explicitly. It can be modified as s1 =(short)(s1 1). short s1 = 1; s1 = 1 is correct.

Sixth, the difference between Overload and Override. Can Overloaded methods change the type of return value?

Overriding and overloading of methods are different manifestations of Java polymorphism. Overriding is a manifestation of polymorphism between parent classes and subclasses, and overloading is a manifestation of polymorphism in a class.

If a method defined in a subclass has the same name and parameters as its parent class, we say that the method is overriding (Overriding). When an object of a subclass uses this method, it will call the definition in the subclass. For it, the definition in the parent class seems to be "shielded".

If multiple methods with the same name are defined in a class, and they have different number of parameters or different parameter types, it is called method overloading. Overloaded method can change the type of return value.

Seventh, what is the difference between error and exception?

error indicates a serious problem when recovery is not impossible but difficult. For example, memory overflow. It is impossible to expect a program to handle such a situation.

exception represents a design or implementation problem. That is, it represents a situation that would never occur if the program were running normally.

Eighth, give me a runtime exception you see most often.

ArithmeticException, ArrayStoreException, 
BufferOverflowException, BufferUnderflowException, 
CannotRedoException, CannotUndoException, 
ClassCastException, CMMException, 
ConcurrentModificationException, DOMException, 
EmptyStackException, IllegalArgumentException, 
IllegalMonitorStateException, IllegalPathStateException, 
IllegalStateException, ImagingOpException, 
IndexOutOfBoundsException, MissingResourceException, 
NegativeArraySizeException, NoSuchElementException, 
NullPointerException, ProfileDataException, 
ProviderException, RasterFormatException, SecurityException, 
SystemException, UndeclaredThrowableException, 
UnmodifiableSetException, UnsupportedOperationException

Ninth, the elements in the Set cannot be repeated, so what method is used to distinguish duplication? Is it using == or equals()? What is the difference between them?

The elements in Set cannot be repeated, so use the iterator() method to distinguish whether they are repeated or not. equals() determines whether two Sets are equal.

The equals() and == methods determine whether the reference value points to the same object. equals() is overridden in the class in order to return a true value when the contents and types of the two separate objects match.

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