Complex Number Multiplication LeetCode solution in Java

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class Solution {
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  public String complexNumberMultiply(String num1, String num2) {
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    int val1 = Integer.parseInt(num1.substring(0, num1.indexOf('+')));
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    int val2 = Integer.parseInt(num1.substring(num1.indexOf('+') + 1, num1.length() - 1));
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    int val3 = Integer.parseInt(num2.substring(0, num2.indexOf('+')));
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    int val4 = Integer.parseInt(num2.substring(num2.indexOf('+') + 1, num2.length() - 1));
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    return "" + (val1 * val3 - val2 * val4) + "+" + (val1 * val4 + val3 * val2) + "i";
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  }
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}