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Core Java

Related conceptExceptions

16 — Exceptions

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▶️ java pkg1core/core18ExceptionsDemo.java

Hierarchy

code
1Throwable2├── Error          (OutOfMemoryError — don't catch)3└── Exception4    ├── RuntimeException   (unchecked: NPE, IAE)5    └── IOException etc.   (checked: must handle)

try / catch / finally

java
1try {2    int[] a = new int[2];3    System.out.println(a[5]);4} catch (ArrayIndexOutOfBoundsException e) {5    System.out.println("Caught: " + e.getMessage());6} finally {7    System.out.println("Always runs");8}

try-with-resources (Java 7+)

java
1try (BufferedReader br = Files.newBufferedReader(path)) {2    return br.readLine();3}   // br.close() called automatically

Always use for files, streams, connections.

throw and throws

java
1void withdraw(double balance, double amt) throws InsufficientFundsException {2    if (amt > balance) throw new InsufficientFundsException("short " + amt);3}
  • throw — raise exception now
  • throws — declare checked exceptions on method

Custom exceptions

java
1class InsufficientFundsException extends Exception {2    InsufficientFundsException(String msg) { super(msg); }3}

Extend Exception for checked, RuntimeException for unchecked.

Best practices

Do Don't
Catch specific types catch (Exception e) {} empty
Chain cause: new X("msg", cause) Swallow exceptions
Use try-with-resources Forget to close files
Fail fast with clear messages Use exceptions for normal flow

Interview drill → 07 Exceptions

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