Java naming conventions
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Please follow the [Java naming conventions][1]. Variables should start with a lowercase character, so it should read (also note the placing of `[]` makes it clearer that stack is an array of `Character`)

    static  Character[] stack= new Character[25]; 

Why maximum stack size?
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There is no given requirement for the limit of the size of the stack, so you should not use a fixed-size data structure like array, but rather opt for something like `List`

     static  List<Character> stack= new ArrayList<>(); 


Separation of concerns
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There are two functionalities in you code directly interwoven. First, there is the 'Stack', then there is the usage of the `Stack`. Move all the `Stack` related functionality to it's own class.

Don't use magic values  
---
If there is an error in the state of the stack (for example, if you try to `pop()` an empty stack, you could throw an `Exception`, for example a `NoSuchElementException()`. Or you can have the Stack of Optional<Character> and return `Optional.empty()`. Or even return `null` if no empty values on the stack are allowed.

Pair the parentheses, separate to own data type
---
You coudl implement a Delimiter enum type, like such

    enum Delimiter
    {
      PARENTHESES( '(', ')' ),
      BRACES     ( '{', '}' ),
      BRACKETS   ( '[', ']' );
      //easily expandable with for example: 〔 〕 – tortoise shell brackets
      
    
      public final char openChar;
      public final char closeChar;
    
      public Delimiter(char openChar, char closeChar)
      {
          this.openChar = openChar;
          this.closeChar = closeChar;
      }
    }

Then, when looping over the characters, you could use:

     for (i = 0; i< str.length(); i++)
     {
         char c = str.charAt(i);
         for (Delimiter delimiter : Delimiter.values())
         {
              if (c == delimiter.openChar)
              {
                  stack.push(delimiter);
              } 
              else if (c == delimiter.closeChar)
              {
                  //pop the stack and check if the closechar matches the openchar of the popped element
    
              }
         }


  [1]: https://www.oracle.com/technetwork/java/codeconventions-135099.html