面试题与答案

'==' is used to compare object references, 'equals()' is used to compare object content, and 'hashCode()' returns the hash code value of an object. It is recommended to override 'equals()' and 'hashCode()' when dealing with custom classes.

Example:

Example:

@Override
public boolean equals(Object obj) { 
  // custom implementation
}

@Override
public int hashCode() { 
  // custom implementation
}

Garbage collection in Java automatically reclaims the memory occupied by objects that are no longer reachable. The 'java.lang.System.gc()' method and the 'finalize()' method are related to garbage collection.

Example:

No specific example, as garbage collection is a background process handled by the JVM.

'transient' is used to indicate that a variable should not be serialized during object serialization. The value of a transient variable is not persisted when the object is stored.

Example:

Example:

class MyClass implements Serializable { 
  transient int sensitiveData; 
}

'volatile' is used to indicate that a variable's value may be changed by multiple threads simultaneously. It ensures that the variable is always read from and written to the main memory, avoiding thread-local caching.

Example:

Example:

volatile boolean flag = true; 
// variable shared among multiple threads

The 'assert' statement is used for debugging purposes to check if a given boolean expression is true. If it's false, an 'AssertionError' is thrown. 'assert' statements can be enabled or disabled during runtime using the '-ea' or '-da' JVM options.

Example:

Example:

int x = -1;
assert x >= 0 : 'x should be non-negative';

The 'classpath' is a parameter in the Java Virtual Machine (JVM) that specifies the location of user-defined classes and packages. It can be set using the '-classpath' or '-cp' option when running Java applications. It includes directories, JAR files, and ZIP archives containing Java classes.

Example:

Example:

java -cp myapp.jar com.example.MyClass

'NaN' (Not a Number) is a special floating-point value used to represent undefined or unrepresentable results of mathematical operations. It is typically the result of operations like 0.0/0.0 or Math.sqrt(-1). 'NaN' is represented using the 'Double.NaN' constant.

Example:

Example:

double result = 0.0 / 0.0; // results in NaN

The 'java.util.concurrent' package in Java provides a framework for concurrent programming. It includes classes and interfaces for thread management, concurrent collections, synchronization utilities, and high-level concurrency abstractions.

Example:

Example:

ExecutorService executor = Executors.newFixedThreadPool(5);
executor.submit(() -> System.out.println('Hello, concurrent world!'));
executor.shutdown();