Java interview questions summarized from many years of development experience - (3)

1. What are the uses of the final keyword in Java?

The modified variables must be initialized or cannot be modified, the modified class cannot be inherited, and the modified method cannot be overridden.

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2. There is a return statement in try{}, then follow this try Will the code in the finally{} be executed? When will it be executed? Before or after return?

Will be executed after return is executed but before the result is returned. return The return result will be placed on the function stack and wait for finally to be executed before it is actually returned;

3. Explain the differences between final, finally, and finalize.

final is used to modify classes (cannot be inherited), variables (cannot be modified, can only be assigned once), methods (cannot be overridden)

finally is used for try{}catch During the execution process, there are finally blocks that must be executed whether there are exceptions to catch, resources to close, etc...

finalize is the method name, the last words of the object, and is used to execute the method before the garbage collector reclaims and cleans the object. Defined by the object class, all classes inherit it.

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4. How to create objects through reflection?

Create an instance of the corresponding class of the Class object through the newInstance() method of the Class object

Use the Class object to obtain the specified Constructor object, and call the newInstance() method of the Constructor object to create it Class objects correspond to instances of classes.

5. New features of Java 8

1. Java 8 introduces the concept of functional interface. Lambda allows functions to be passed as arguments to a method, or code to be treated as data.

2. Default methods and static methods of the interface. Define the default method in the interface, use the default keyword, and provide a default implementation. All classes that implement this interface will accept the implementation of the default method, unless the subclass provides its own implementation. You can also provide an implementation by defining a static method in the interface and using the static keyword.

3. Method reference, combined with Lambda expression

(1) Constructor reference. The syntax is Class::new

(2) Static method reference. The syntax is Class::static_method

(3) Any object method reference of a specific class. Its syntax is Class::method

(4) The method reference of a specific object, its syntax is instance::method

4. Java 8 introduces duplicate annotations, and the same annotations are in the same Places can be declared multiple times. The repeat annotation mechanism itself needs to be annotated with @Repeatable. Java 8 has made optimizations at the compiler layer, and the same annotations will be saved in a collection, so the underlying principle has not changed.

5. Extended support for annotations. Java 8 has expanded the context of annotations. Annotations can be added to almost anything, including local variables, generic classes, parent classes and interface implementations, and even method exceptions. Add annotations.

6. Introduce the Optional class to prevent null pointer exceptions. The Optional class is actually a container: it can save values ​​of type T or null. Using the Optional class we don't need to explicitly check the null pointer.

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7. Introduce Stream API and functional programming style to make the code serial and support continuous and parallel aggregation operations. easy to understand.

8. JavaScript engine Nashorn. Nashorn allows JavaScript applications to be developed and run on the JVM, allowing Java and JavaScript to call each other.

9. Base64, the Base64 class provides URL and MIME-friendly encoders and decoders.

10. Date/Time API (JSR 310) provides a new java.time package, which can be used to replace java.util.Date and java.util.Calendar. Generally, Clock, LocaleDate, Classes such as LocalTime, LocaleDateTime, ZonedDateTime, and Duration are very good at improving time and date.

In addition to these ten new features, there are other new features:

• Better type speculation mechanism: Java 8 has type speculation A big improvement, this makes the code cleaner and does not require too many casts.

• Compiler optimization: Java 8 adds the parameter name of the method to the bytecode, so that the parameter name can be obtained through reflection at runtime. You only need to use - at compile time. parameters parameters.

• Parallel (parallel) array: Supports parallel processing of arrays, mainly the parallelSort() method, which can greatly improve the speed of array sorting on multi-core machines.

• Concurrency: Based on the new Stream mechanism and Lambda, some new methods have been added to support aggregation operations.

• Nashorn engine jjs: A command line tool based on Nashorn engine. It accepts some JavaScript source code as parameters and executes these source codes.

• Class dependency analyzer jdeps: can display package-level or class-level dependencies of Java classes.

• The JVM's PermGen space was removed: it was replaced by Metaspace (JEP 122).

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