1. The difference between int and Integer

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1. Integer is a wrapper class of int, which is a basic data type of java

2. Integer variables must be instantiated before they can be used, while int variables do not need to be

3 , Integer is actually a reference to an object. When a new Integer is created, a pointer is actually generated pointing to the object; while int directly stores the data value

4. The default value of Integer is null, and the default value of int is The value is 0

2. What is a for each loop? What data types can it loop through?

The enhanced version of the for loop is simpler to write and less error-prone (because you don’t have to worry about the array going out of bounds). The bottom layer is also obtained using an iterator, but the iterator is obtained by the jvm. We only need to obtain the iterator, so in the process of using the foreach loop variable element, we are not allowed to use the collection object to modify the number of elements in the collection.

Writing:

for(String it : set){
    System.out.println("集合的元素：" + it);
}

Disadvantages:

An error may be reported when operating the array index or adding or deleting the collection.

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3. What is the difference between overloading and rewriting?

1. Method overloading

is a means for classes to process different types of data in a unified way. Multiple functions with the same name exist at the same time, with different number/type of parameters. Overloading is a manifestation of polymorphism in a class.

Method overloading in Java means that you can create multiple methods in a class. They have the same name but different parameters and different definitions.

When calling methods, the specific number and parameter types passed to them are used to determine which method to use. This is polymorphism.

When overloading, 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 value type cannot be used as a criterion for distinguishing overloaded functions.

2. Method rewriting

The parameter list must be completely the same as the overridden method, otherwise it cannot be called rewriting but overloading.

The returned type must always be the same as the return type of the overridden method, otherwise it cannot be called an override but an overload.

The access modifier limit must be greater than the access modifier of the overridden method (public>protected>default>private)

The overridden method must not throw a new checked exception or be larger than The overridden method declares a broader checked exception. For example: a method of the parent class declares a checked exception IOException. When overriding this method, you cannot throw Exception. You can only throw exceptions of subclasses of IOException, and you can throw unchecked exceptions.

Note: The constructor cannot be inherited, so it cannot be overridden, but it can be overloaded.

4. What is the difference between interface and abstract class?

1. Neither abstract classes nor interfaces can be instantiated directly. If they are to be instantiated, abstract class variables must point to subclass objects that implement all abstract methods, and interface variables must point to classes that implement all interface methods. object.

2. Abstract classes must be inherited by subclasses, and interfaces must be implemented by classes.

3. Interfaces can only be used for method declarations. Abstract classes can be used for method declarations and method implementations.

4. Variables defined in interfaces can only be public static constants. Variables in abstract classes are ordinary variables.

5. All abstract methods in an abstract class must be implemented by the subclass. If the subclass cannot implement all the abstract methods of the parent class, then the subclass can only be an abstract class. Similarly, when a class implements an interface, if it cannot implement all interface methods, then the class can only be an abstract class.

6. Abstract methods can only be declared, not implemented. Interfaces are the result of design, and abstract classes are the results of reconstruction.

7. Abstract classes do not need abstract methods.

8. If there is an abstract method in a class, then the class can only be an abstract class

9. The abstract method must be implemented, so it cannot be static or private.

10. Interfaces can inherit interfaces and multiple interfaces, but classes can only inherit from a single root.

The main reason why multiple inheritance of classes is not allowed is that if A inherits B and C at the same time, and b and c have a D method at the same time, how does A decide which one to inherit?

But there is no such problem with interfaces. Interfaces are all abstract methods and it doesn't matter who inherits them, so interfaces can inherit multiple interfaces.

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5. The difference between final, finally, finalize

final: Keywords and modifiers in java.

A) If a class is declared final, it means that it cannot derive new subclasses and cannot be inherited as a parent class. Therefore, a class cannot be declared abstract and final at the same time.

B) If you declare variables or methods as final, you can ensure that they will not be changed during use.

（1）被聲明為final的變量必須在聲明時(shí)給定初值，而在以后的引用中只能讀取，不可修改。

（2）被聲明final的方法只能使用，不能重載。

finally：java的一種異常處理機(jī)制。

finally是對(duì)Java異常處理模型的最佳補(bǔ)充。finally結(jié)構(gòu)使代碼總會(huì)執(zhí)行，而不管無(wú)異常發(fā)生。使用finally可以維護(hù)對(duì)象的內(nèi)部狀態(tài)，并可以清理非內(nèi)存資源。特別是在關(guān)閉數(shù)據(jù)庫(kù)連接這方面，如果程序員把數(shù)據(jù)庫(kù)連接的close()方法放到finally中，就會(huì)大大降低程序出錯(cuò)的幾率。

finalize，它是一個(gè)方法，屬于java.lang.Object類，它的定義如下：

protected void finalize()throws Throwable{}

眾所周知，finalize()方法是GC（garbagecollector運(yùn)行機(jī)制的一部分,在此我們只說(shuō)說(shuō)finalize()方法的作用是什么呢？

finalize()方法是在GC清理它所從屬的對(duì)象時(shí)被調(diào)用的，如果執(zhí)行它的過(guò)程中拋出了無(wú)法捕獲的異常（uncaughtexception，GC將終止對(duì)改對(duì)象的清理，并且該異常會(huì)被忽略；直到下一次GC開(kāi)始清理這個(gè)對(duì)象時(shí)，它的finalize()會(huì)被再次調(diào)用。

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