nd4j-backends/nd4j-api-parent/nd4j-api/src/main/java/org/nd4j/linalg/indexing/BooleanIndexing.java
public static INDArray firstIndex(INDArray array, Condition condition) {
        if (!(condition instanceof BaseCondition))
            throw new UnsupportedOperationException("Only static Conditions are supported");

        FirstIndex idx = new FirstIndex(array, condition);
        Nd4j.getExecutioner().exec(idx);
        return Nd4j.scalar((double) idx.getFinalResult());
    }
public static INDArray firstIndex(INDArray array, Condition condition) {
        if (!(condition instanceof BaseCondition))
            throw new UnsupportedOperationException("Only static Conditions are supported");

        FirstIndex idx = new FirstIndex(array, condition);
        Nd4j.getExecutioner().exec(idx);
        return Nd4j.scalar((double) idx.getFinalResult());
    }
This method returns first index matching given condition

PLEASE NOTE: This method will return -1 value if condition wasn't met

@param array
@param condition
@return