Java Notes
HubArray IntroductionArray OperationsArray Programs — FundamentalsArray Interview QuestionsJagged Array in JavaMulti Dimensional ArrayOne Dimensional Arraybreak and continue in JavaDo-While Loop in JavaFor Loop in JavaIf-Else Statements in JavaNested Loops in JavaSwitch-Case in JavaWhile Loop in JavaJava Compilation ProcessFeatures of JavaYour First Java ProgramHistory of JavaJava EditionsJava Program StructureJDK, JRE, and JVMWhat is Java?Immutable Strings in JavaString Class in JavaString Interview QuestionsString MethodsString Programs — FundamentalsStringBuffer in JavaStringBuilder in JavaComments in JavaData Types in JavaIdentifiers in JavaInput and Output in JavaKeywords in JavaOperators in JavaType Casting in JavaVariables in JavaAbstract Class in JavaAbstraction in JavaAnonymous Class in JavaClass and Object in JavaConstructor in JavaEncapsulation in JavaInheritance in JavaInner Class in JavaInterface in JavaMethod Overloading in JavaMethod Overriding in JavaObject Class in JavaObject Cloning in JavaOOPs IntroductionPolymorphism in Javastatic Keyword in Javasuper Keyword in Javathis Keyword in JavaWrapper Class in JavaCollections Framework OverviewCollections Practice ProgramsComparable vs Comparator in JavaGenerics in JavaIterator in JavaArrayList in JavaLinkedList in JavaStack in Java — Collections FrameworkVector in JavaHashMap in JavaHashtable in JavaLinkedHashMap in JavaTreeMap in JavaArrayDeque in JavaDeque Interface in JavaPriorityQueue in JavaHashSet in JavaLinkedHashSet in JavaTreeSet in JavaException Handling Best PracticesCustom Exceptions in JavaException Hierarchy in JavaFinally Block in JavaException Handling Introductionthrow and throws Keywords in JavaTry-Catch Block in JavaBuffered Streams in JavaByte Streams in JavaCharacter Streams in JavaDeserialization in JavaFile Class in JavaJava NIO PackageSerialization in JavaDate and Time API in Java 8Default and Static Methods in InterfacesFunctional Interface in Java 8Lambda Expression in Java 8Method Reference in Java 8Optional Class in Java 8Stream API in Java 8Creating Threads in JavaDaemon Threads in JavaExecutor Framework in JavaInter-Thread Communication in JavaMultithreading Programs in JavaRunnable Interface in JavaSynchronization in JavaThread Class in JavaThread IntroductionThread Life Cycle in JavaThread Priority in JavaBatch Processing in JDBCCallableStatement in JDBCJDBC ArchitectureJDBC IntroductionJDBC ProjectsMySQL Connection with JDBCPreparedStatement in JDBCResultSet in JDBCStatement Interface in JDBCTransaction Management in JDBCCookies in ServletsExpression Language (EL)GenericServletHttpServletJSP IntroductionJSP Tags and JSTLMVC ArchitectureRequest and Response ObjectsServlet IntroductionServlet Life CycleSession TrackingDependency InjectionAPI GatewayConfig ServerDocker DeploymentEureka Server - Service DiscoverySpring BeansCRUD ApplicationException Handling in Spring BootJWT Authentication — Java ProgrammingREST API with Spring BootSpring SecuritySpring Boot IntroductionSpring Data JPAValidation in Spring BootSpring Core ContainerSpring Framework IntroductionDynamic Programming in JavaGraph Data Structure in JavaHashing in JavaHeap (DSA)Linked List in JavaQueue in JavaRecursion in JavaSearching Algorithms in JavaSorting Algorithms in JavaStack in Java — DSA in JavaTime ComplexityTree Data Structure in JavaAdapter Design PatternBuilder Design PatternFactory Design PatternMVC Pattern — Java ProgrammingObserver Design PatternSingleton Design PatternArray Programs — PracticeBasic Java ProgramsCollection ProgramsJDBC ProgramsMultithreading ProgramsOOP ProgramsPlacement Coding QuestionsString Programs — PracticeBanking System — Java ProgrammingChat ApplicationE-Commerce BackendEmployee Management SystemLibrary Management System — Java ProgrammingSpring Boot Fullstack ProjectStudent Management System — Java ProgrammingCoding Interview QuestionsCollections Interview QuestionsCore Java Interview QuestionsJDBC Interview QuestionsMultithreading Interview QuestionsOOPs Interview QuestionsSpring Framework Interview QuestionsJava Cheat SheetImportant Formulas & ConceptsImportant Java ProgramsQuick Revision Notes — Java ProgrammingJava Master Course - Learning Roadmap
Complete guide to Stack data structure — LIFO principle, array and linked list implementations, applications, expression evaluation, and classic problems.
What is a Stack?
A Stack is a LIFO (Last In, First Out) data structure. The last element added is the first one removed — like a stack of plates.
| push(4): [1, 2, 3] | [1, 2, 3, 4] (add to top) |
| pop(): [1, 2, 3, 4] | [1, 2, 3] (remove from top, returns 4) |
| peek(): [1, 2, 3] | returns 3 (view top without removing) |
| All operations | O(1) |
Applications
Balanced Parentheses
Example
public static boolean isBalanced(String s) {
Stack<Character> stack = new Stack<>();
for (char c : s.toCharArray()) {
if (c == '(' || c == '{' || c == '[') {
stack.push(c);
} else {
if (stack.isEmpty()) return false;
char open = stack.pop();
if ((c == ')' && open != '(') ||
(c == '}' && open != '{') ||
(c == ']' && open != '[')) return false;
}
}
return stack.isEmpty();
}
Infix to Postfix Conversion
java example
public static String infixToPostfix(String infix) {
StringBuilder result = new StringBuilder();
Stack<Character> stack = new Stack<>();
for (char c : infix.toCharArray()) {
if (Character.isLetterOrDigit(c)) {
result.append(c);
} else if (c == '(') {
stack.push(c);
} else if (c == ')') {
while (!stack.isEmpty() && stack.peek() != '(')
result.append(stack.pop());
stack.pop(); // Remove '('
} else { // Operator
while (!stack.isEmpty() && precedence(stack.peek()) >= precedence(c))
result.append(stack.pop());
stack.push(c);
}
}
while (!stack.isEmpty()) result.append(stack.pop());
return result.toString();
}
private static int precedence(char op) {
switch (op) { case '+': case '-': return 1; case '*': case '/': return 2; case '^': return 3; }
return -1;
}Next Greater Element
Example
public static int[] nextGreater(int[] arr) {
int n = arr.length;
int[] result = new int[n];
Stack<Integer> stack = new Stack<>(); // Stores indices
for (int i = n - 1; i >= 0; i--) {
while (!stack.isEmpty() && arr[stack.peek()] <= arr[i])
stack.pop();
result[i] = stack.isEmpty() ? -1 : arr[stack.peek()];
stack.push(i);
}
return result;
}
// [4, 5, 2, 10, 8] → [5, 10, 10, -1, -1]
Min Stack (O(1) getMin)
Example
class MinStack {
private Stack<Integer> main = new Stack<>();
private Stack<Integer> min = new Stack<>();
public void push(int val) {
main.push(val);
if (min.isEmpty() || val <= min.peek()) min.push(val);
}
public int pop() {
int val = main.pop();
if (val == min.peek()) min.pop();
return val;
}
public int getMin() { return min.peek(); } // O(1)
public int top() { return main.peek(); }
}
Interview Questions
Q1: How would you implement a stack using queues?
Answer: Use two queues. For push: add to q1. For pop: move all but last from q1 to q2, remove last from q1, swap q1 and q2. Alternative: single queue, after enqueue rotate n-1 elements.
Q2: How do you sort a stack using only another stack?
Answer: Pop temp from source. While dest has larger elements on top, pop them back to source. Push temp to dest at correct position. Repeat. Time: O(n²).
Q3: What is the difference between Stack class and Deque in Java?
Answer: java.util.Stack extends Vector (synchronized, legacy). Use Deque<Integer> stack = new ArrayDeque<>() instead — faster (no synchronization), preferred per Java docs. Use push()/pop()/peek().
Q4: Design a browser back/forward button.
Answer: Two stacks: backStack and forwardStack. Visit new page: push current to backStack, clear forwardStack. Back: push current to forwardStack, pop backStack. Forward: push current to backStack, pop forwardStack.
Q5: What is the stock span problem?
Answer: For each day, find how many consecutive previous days had price ≤ today. Use a stack storing indices of days with higher prices. For each day, pop smaller elements — span is current index minus stack top index.
Exam Focus
Revise definitions, diagrams, examples, and short-answer points for Stack in Java — DSA in Java.
Interview Use
Prepare one clear explanation, one practical example, and one common mistake for this Java Master Course topic.
Search Terms
java-master-course, java master course, java, master, course, dsa, stack, stack in java — dsa in java
Related Java Master Course Topics
Continue learning this concept
DSA in JavaDynamic Programming in JavaComplete guide to Dynamic Programming in Java covering memoization, tabulation, classical DP problems (Fibonacci, Knapsack, LCS, LIS, Coin Change), time/space optimization, and interview patterns.DSA in JavaGraph Data Structure in JavaComplete guide to graphs — representation, BFS, DFS, shortest path algorithms, topological sort, and classic graph problems in Java.DSA in JavaHashing in JavaComplete guide to hashing — hash functions, collision resolution, HashMap internals, custom hashCode/equals, and applications of hashing in problem-solving.DSA in JavaHeap (DSA)Complete guide to Heap data structure in Java covering Min Heap, Max Heap, heap operations, heapify algorithm, PriorityQueue class, heap sort, and applications with detailed implementations.DSA in JavaLinked List in JavaComplete guide to Linked Lists — Singly, Doubly, and Circular linked lists with Java implementations, operations, and classic interview problems.DSA in JavaQueue in JavaComplete guide to Queue data structure — FIFO principle, circular queue, priority queue, deque, BFS applications, and classic problems.