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This article mainly introduces "what are the basic interview questions of Java". In the daily operation, I believe that many people have doubts about the basic interview questions of Java. The editor consulted all kinds of materials and sorted out simple and easy-to-use methods of operation. I hope it will be helpful for you to answer the doubts of "what are the basic interview questions of Java?" Next, please follow the editor to study!

What's the difference between 1.JDK and JRE?

A: JRE is a java runtime environment that includes java virtual machines and java basic class libraries. Is the software environment needed for programs written in java language to run, and is provided for users who want to run Java programs.

JDK is a java development kit, which is needed by programmers to write java programs in Java language, and is provided to programmers.

2.What is the difference between equals = and QR?

Answer: = = is the operator that compares the address or basic type of two objects. Equals is a method in Object that compares the contents of two objects.

3. If the hashCode () of two objects is the same, then equals () must also be true, right?

A: no! Just because hashCode () is the same does not mean that two objects are the same. The hashCode value is obtained from the hash table, hash is a function, the implementation of this function is an algorithm, through the hash algorithm to calculate the hash value, the hash table is composed of hash values, a total of 8 locations.

1) if two objects equals,Java the runtime environment will think that their hashcode must be equal.

2) if two objects are not equals, their hashcode is likely to be equal.

3) if two objects have the same hashcode, they are not necessarily equals.

4) if the hashcode of two objects is not equal, they must not equals.

What is the role of 4.final in java? What is the difference with finally and finalize?

A: the role of final varies depending on the type of modification:

1) final modifies a property or variable in a class: the role of final, regardless of whether the attribute is a base type or a reference type.

2) the "value" stored in the variable modified by final cannot be changed. 3) final modifies the methods in the class: can be inherited, but cannot be overridden after inheritance. 4), final decorated class: class cannot be inherited. 5) finally is part of the structure of the exception handling statement, indicating that it is always executed.

6) finalize is a method of the Object class. This method of the recycled object is called when the garbage collector executes, which can override other resource collection when this method provides garbage collection, such as closing files.

How much is Math.round (- 1.5) equal to in 5.java and how much is Math.round (1.5) equal?

Answer: equal to-1, equal to 2

Calculation method:

1), the first place 5 after the decimal point of the parameter, the operation result is the absolute value of the integer part of the parameter + 1, and the symbol (that is, positive or negative) remains unchanged.

3), the first place after the decimal point of the parameter is 5, the result of positive number operation is integer part + 1, and the result of negative number operation is integer part.

Summary: add all more than five, equal to five positive numbers, and do not add less than five.

6. What is the difference between overloading and rewriting in Java?

Both overloading and overriding allow you to implement different functions with the same name, but overloading is a compile-time activity and rewriting is a run-time activity. You can override methods in the same class, but you can only override methods in subclasses. Rewriting must have inheritance.

Rewrite:

1) the methods inherited from the base class can be overridden as needed in the subclass.

2) the overridden method and the overridden method must have the same method name, parameter list, and return type.

3) overridden methods cannot use more stringent access than overridden methods.

When overloaded, the method name should be the same, but the parameter type and number are different, and the return value type can be the same or different. The return type cannot be used as a distinguishing criterion for overloaded functions.

What are the classes of operation strings in 7.java? What's the difference between them?

Answer: String, StringBuffer and StringBuilder can operate to concatenate strings.

Difference: String is an immutable object, and a new object is generated every time you change the type of String. StringBuffer and StringBuilder can change the object.

For operational efficiency: StringBuilder > StringBuffer > String

For thread safety: StringBuffer is thread-safe and can be used for multithreading; StringBuilder is non-thread-safe for single thread

Is 8.String str= "I" the same as String str=new String ("I")?

A: it is different because they are not the same object.

9. Access the modifier public,private,protected, and what is the difference when not writing (default)?

Answer: when members of the class do not write access decorations, the default is default. The default is public for other classes in the same package and private for other classes that are not in the same package. Protected is equivalent to being public to subclasses and private to classes that are not in the same package without parent-child relationships. In Java, the modifiers for external classes can only be public or default, and the modifiers for members of a class (including inner classes) can be the above four.

To put it simply: Protected can be accessed in-package and out-of-package subclasses, default can only be accessed in the same package, and prvate can only be accessed in the same class.

What are the basic types of 10.java? Is String a basic type?

There are only eight basic data types in Java: byte, short, int, long, float, double, char, boolean;String is an object, not a primitive type.

What's the difference between 11.int and Integer, and why do you use wrapper classes?

A: in order to manipulate these basic data types as objects, Java introduces a corresponding wrapper type for each basic data type. For example, the wrapper class of int is that Integer,Java provides a wrapper type for each primitive type:

-original type: boolean,char,byte,short,int,long,float,double

-Packaging type: Boolean,Character,Byte,Short,Integer,Long,Float,Double

twelve。 What's the difference between abstract classes (abstract class) and interfaces (interface)?

1) Abstract classes and interfaces cannot be instantiated and cannot have constructors

2). The interface is modified with interface

3) A class can implement one or more interfaces, but can only inherit one abstract class

4), the methods in the interface can only be abstract methods, class methods or default methods, and the methods in the interface cannot be implemented by methods, but both class methods and default methods must be implemented.

5) using abstract classes is for reuse. Reduce the amount of coding and reduce the coupling.

13. What are the three characteristics of object-oriented? What is encapsulation, inheritance, and polymorphism?

Answer: three basic characteristics of object-oriented: encapsulation, inheritance and polymorphism

1) Encapsulation (English: Encapsulation) is a method that wraps and hides the implementation details of an abstract function interface. Encapsulation can be thought of as a protective barrier to prevent code and data of this class from being randomly accessed by code defined by external classes.

2), inheritance can make a subclass have the properties and methods of the parent class, or redefine or append properties and methods, etc., inheritance can be understood as the process of one object getting properties from another object.

3) Polymorphism is the ability of the same behavior to have many different forms or forms. Polymorphism is the embodiment of many forms of expression of the object. For example, when we say "pet", it has many different expressions or implementations, such as kittens, puppies, lizards and so on. Then I go to the pet store and say, "Please give me a pet." the waiter can give me a kitten, a dog or a lizard, and we will say that the "pet" object is polymorphic.

14. List some of your common runtime exceptions?

Answer:-ClassNotFoundException (class cannot find exception)

-ClassCastException (class conversion exception)

-IllegalArgumentException (illegal parameter exception)

-IndexOutOfBoundsException (subscript out of bounds exception)

-NullPointerException (null pointer exception)

-SecurityException (security exception)

What are the 15.Java containers? What is the difference between Collection, Collections, List, Set and Map?

Answer: Collection interface and Map interface. List and Set inherit the Collection interface. Collections is a utility class that provides a series of static methods to assist container operations, including container search, sorting, thread safety, and so on.

1) List is an ordered collection that allows repetition. ArrayList, LinkedList and Vector implement the List interface, in which ArrayList is easy to query and random access, while LinkedList is a linked list and easy to insert and delete. The method in Vector adds synchronized decoration, so Vector is a thread-safe container, but its performance is worse than ArrayList, so it is already a legacy container in Java.

2) while Set is an unordered set, repetition is not allowed; the implementation classes of Set are EnumSet, SortedSet, HashSet

Among them, HashSet has excellent query and lookup performance, and its subclass is LinkedHashSet, which accesses the elements in the collection through the internal order maintained by the linked list and the order in which the elements are added, and uses the linked list to record the adding order of the collection; TreeSet is a subclass of SortedSet, uses the data structure of the red-black tree to store the collection elements, supports natural sorting and custom sorting, and is an ordered collection.

Note: Hashtable, Dictionary, BitSet, Stack and Properties are all legacy containers) and are no longer recommended.

3), Map interface key-value pair (key-value pair) mapping Map collection class is used to store element pairs (called "keys" and "values"), where each key maps to a value.

16.hashMap principle, changes made by java8

Answer: in terms of structural implementation, HashMap is implemented by array + linked list + red-black tree (JDK1.8 adds the red-black tree part). HashMap allows at most the key of one record to be null and the value of multiple records to be null. HashMap is not thread safe. ConcurrentHashMap is thread safe. Solve the collision: when there is a conflict, using the zipper method, the nodes whose keywords are synonyms are linked in a single linked list, and the hash table is long m, then define a pointer array T composed of m head pointers, and the node with the address I is inserted into the single linked list with T (I) as the head pointer. In Java8, the conflicting elements exceed the limit (8) and replace the linked list with a red-black tree.

The difference between 17.HashMap and Hashtable.

A: HashMap is a lightweight implementation of Hashtable (a non-thread-safe implementation). They all complete the Map interface. The main difference is that HashMap allows null (null) keys (key), which may be more efficient than Hashtable because of non-thread safety.

1), HashMap allows empty key-value pairs, but there is only one empty object at most, but HashTable does not. HashMap removed Hashtable's contains method and changed it to containsvalue and containsKey. Because the contains method is easily misleading.

2), Hashtable inherits from the Dictionary class, and HashMap inherits from abstractMap

3) the biggest difference is that the method of Hashtable is Synchronize, while HashMap is not. When multiple threads access Hashtable, they do not need to synchronize their methods themselves, and HashMap must provide external synchronization for them.

HashTable is synchronized, while HashMap is out of sync, which is more efficient than HashTable.

4), Hashtable and HashMap all use the same hash/rehash algorithm, so there won't be much difference in performance.

18. Common data structures.

A: sets, linear structures (arrays, queues, linked lists and stacks), tree structures, graph structures.

What is the difference between 19.Comparator and Comparable?

Answer: the Comparable API is used to define the natural order of objects, which is a sorting interface, while comparator is usually used to define a user-customized order, which is a comparison interface.

What is the difference between a stack and a stack in 20.Java?

A: the heap and stack in JVM belong to different memory areas and are used for different purposes. The stack is often used to hold method frames and local variables, while objects are always allocated on the heap. The stack is usually smaller than the heap and is not shared among multiple threads, while the heap is shared by all threads of the entire JVM.

What are the districts of 21.JVM and what does each district do?

1), method method: shared by all threads. The method area contains all the class information and static variables.

2), heap: shared by all threads, storing object instances and arrays. Java heap is the main area of GC.

3), stack: each thread contains a stack area, and some local variables are stored in the stack.

4) Program counter: a line indicator of the bytecode executed by the current thread.

twenty-two。 Explain what heap space and GC are?

A: when a Java process occurs, memory is allocated to it. Part of the memory is used to create heap space, and when an object is created in the program, memory is allocated from the pair space. GC is a process within JVM that reclaims memory from invalid objects for future allocation.

At this point, the study of "what are the basic interview questions of Java" is over. I hope to be able to solve your doubts. The collocation of theory and practice can better help you learn, go and try it! If you want to continue to learn more related knowledge, please continue to follow the website, the editor will continue to work hard to bring you more practical articles!

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